Do not exit on some non-fatal errors:

- writev() fails in net_output(). The result is a lost packet or packets.
- writev() fails in console_output(). The result is partially lost console
output.
- readv() fails in net_input(). The result is a lost packet or packets.

Rather than bringing the guest down, this patch ignores e.g. an allocation
failure on the host side. Example:

lguest: page allocation failure. order:4, mode:0x4d0
Pid: 4045, comm: lguest Tainted: G        W   2.6.36 #1
Call Trace:
 [<c138d614>] ? printk+0x18/0x1c
 [<c106a4e2>] __alloc_pages_nodemask+0x4d2/0x570
 [<c1087954>] cache_alloc_refill+0x2a4/0x4d0
 [<c1305149>] ? __netif_receive_skb+0x189/0x270
 [<c1087c5a>] __kmalloc+0xda/0xf0
 [<c12fffa5>] __alloc_skb+0x55/0x100
 [<c1305519>] ? net_rx_action+0x79/0x100
 [<c12fafed>] sock_alloc_send_pskb+0x18d/0x280
 [<c11fda25>] ? _copy_from_user+0x35/0x130
 [<c13010b6>] ? memcpy_fromiovecend+0x56/0x80
 [<c12a74dc>] tun_chr_aio_write+0x1cc/0x500
 [<c108a125>] do_sync_readv_writev+0x95/0xd0
 [<c11fda25>] ? _copy_from_user+0x35/0x130
 [<c1089fa8>] ? rw_copy_check_uvector+0x58/0x100
 [<c108a7bc>] do_readv_writev+0x9c/0x1d0
 [<c12a7310>] ? tun_chr_aio_write+0x0/0x500
 [<c108a93a>] vfs_writev+0x4a/0x60
 [<c108aa21>] sys_writev+0x41/0x80
 [<c138f061>] syscall_call+0x7/0xb
Mem-Info:
DMA per-cpu:
CPU    0: hi:    0, btch:   1 usd:   0
Normal per-cpu:
CPU    0: hi:  186, btch:  31 usd:   0
HighMem per-cpu:
CPU    0: hi:  186, btch:  31 usd:   0
active_anon:134651 inactive_anon:50543 isolated_anon:0
 active_file:96881 inactive_file:132007 isolated_file:0
 unevictable:0 dirty:3 writeback:0 unstable:0
 free:91374 slab_reclaimable:6300 slab_unreclaimable:2802
 mapped:2281 shmem:9 pagetables:330 bounce:0
DMA free:3524kB min:64kB low:80kB high:96kB active_anon:0kB inactive_anon:8kB active_file:8760kB inactive_file:2760kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15868kB mlocked:0kB dirty:0kB writeback:0kB mapped:16kB shmem:0kB slab_reclaimable:88kB slab_unreclaimable:148kB kernel_stack:40kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 865 2016 2016
Normal free:150100kB min:3728kB low:4660kB high:5592kB active_anon:6224kB inactive_anon:15772kB active_file:324084kB inactive_file:325944kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:885944kB mlocked:0kB dirty:12kB writeback:0kB mapped:1520kB shmem:0kB slab_reclaimable:25112kB slab_unreclaimable:11060kB kernel_stack:1888kB pagetables:1320kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 0 9207 9207
HighMem free:211872kB min:512kB low:1752kB high:2992kB active_anon:532380kB inactive_anon:186392kB active_file:54680kB inactive_file:199324kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:1178504kB mlocked:0kB dirty:0kB writeback:0kB mapped:7588kB shmem:36kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 0 0 0
DMA: 3*4kB 65*8kB 35*16kB 18*32kB 11*64kB 9*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 3524kB
Normal: 35981*4kB 344*8kB 158*16kB 28*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 150100kB
HighMem: 5732*4kB 5462*8kB 2826*16kB 1598*32kB 84*64kB 10*128kB 7*256kB 1*512kB 1*1024kB 1*2048kB 9*4096kB = 211872kB
231237 total pagecache pages
2340 pages in swap cache
Swap cache stats: add 160060, delete 157720, find 189017/194106
Free swap  = 4179840kB
Total swap = 4194300kB
524271 pages RAM
296946 pages HighMem
5668 pages reserved
867664 pages shared
82155 pages non-shared
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>