Commit 4fc9b87b ("net: fs_enet: Implement NETIF_F_SG feature")
brings a trouble to Freescale MPC512x: a kernel oops happens
during sending non-linear sk_buff with .data not aligned by 4.

Log quotation:

Unable to handle kernel paging request for data at address 0xe467c000
Faulting instruction address: 0xc000cd44
Oops: Kernel access of bad area, sig: 11 [#1]
MPC512x generic
Modules linked in:
CPU: 0 PID: 984 Comm: kworker/0:1H Not tainted 4.1.0-rc8-00024-gbb16140a #2
Workqueue: rpciod rpc_async_schedule
task: cf364a50 ti: cf362000 task.ti: cf362000
NIP: c000cd44 LR: c000c720 CTR: 00000206
REGS: cf363ac0 TRAP: 0300   Not tainted  (4.1.0-rc8-00024-gbb16140a)
MSR: 00009032 <EE,ME,IR,DR,RI>  CR: 42004082  XER: 00000000
DAR: e467c000 DSISR: 20000000
GPR00: c0279e24 cf363b70 cf364a50 e467c000 00000206 0000001f 00000001 00000001
GPR08: 00000000 e467c000 e46800be 000139a6 82002082 00000000 c002e46c cf3c3680
GPR16: c044cb30 c04b0000 cf363c48 00000000 00000001 fde0315c 00000000 0000000b
GPR24: 0000002c 000040be cf339aa0 0000000b 00000001 cf873210 00282f85 00000000
NIP [c000cd44] clean_dcache_range+0x1c/0x30
LR [c000c720] dma_direct_map_page+0x40/0x94
Call Trace:
[cf363b70] [cf339b60] 0xcf339b60 (unreliable)
[cf363b90] [c0279e24] fs_enet_start_xmit+0x1c8/0x42c
[cf363bd0] [c02ff710] dev_hard_start_xmit+0x2dc/0x3d4
[cf363c40] [c0319c60] sch_direct_xmit+0xcc/0x1cc
[cf363c70] [c02ff9c0] __dev_queue_xmit+0x1b8/0x47c
[cf363ca0] [c032a3e8] ip_finish_output+0x1fc/0x9a8
[cf363ce0] [c032c31c] ip_send_skb+0x1c/0xa4
[cf363cf0] [c035112c] udp_send_skb+0xe4/0x2e8
[cf363d10] [c0351368] udp_push_pending_frames+0x38/0x84
[cf363d20] [c03537b8] udp_sendpage+0x134/0x174
[cf363d70] [c0384fd4] xs_sendpages+0x21c/0x250
[cf363db0] [c03852bc] xs_udp_send_request+0x50/0xf8
[cf363de0] [c0382f08] xprt_transmit+0x64/0x280
[cf363e20] [c038017c] call_transmit+0x168/0x234
[cf363e40] [c0387918] __rpc_execute+0x88/0x2b0
[cf363e80] [c00296f8] process_one_work+0x124/0x2fc
[cf363ea0] [c0029a00] worker_thread+0x130/0x480
[cf363ef0] [c002e528] kthread+0xbc/0xd0
[cf363f40] [c000e4a8] ret_from_kernel_thread+0x5c/0x64
Instruction dump:
7c70faa6 60630800 7c70fba6 4c00012c 4e800020 38a0001f 7c632878 7c832050
7c842a14 5484d97f 4d820020 7c8903a6 <7c00186c> 38630020 4200fff8 7c0004ac
---[ end trace c846c1eceb513c85 ]---

The reason:

MPC5121 FEC requires 4-byte alignment for TX data buffer and calls
tx_skb_align_workaround() for copying sk_buff with not aligned .data to a new
sk_buff with aligned one. But tx_skb_align_workaround() uses
skb_copy_from_linear_data() which doesn't work for non-linear sk_buff:
a new sk_buff has non-zero nr_frags and zero .data_len.

So improve the condition of calling tx_skb_align_workaround() and use
skb_linearize() in it.
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>