__get_request() can call trace_block_getrq() with bio=NULL which causes
block_get_rq::TP_fast_assign() to deref a NULL pointer and panic.

Syzkaller fuzzer panics with
linux-next (1d53d908b79d7870d89063062584eead4cf83448):
  kasan: GPF could be caused by NULL-ptr deref or user memory access
  general protection fault: 0000 [#1] SMP KASAN
  Modules linked in:
  CPU: 0 PID: 2983 Comm: syzkaller401111 Not tainted 4.13.0-rc7-next-20170901+ #13
  task: ffff8801cf1da000 task.stack: ffff8801ce440000
  RIP: 0010:perf_trace_block_get_rq+0x697/0x970 include/trace/events/block.h:384
  RSP: 0018:ffff8801ce4473f0 EFLAGS: 00010246
  RAX: ffff8801cf1da000 RBX: 1ffff10039c88e84 RCX: 1ffffd1ffff84d27
  RDX: dffffc0000000001 RSI: 1ffff1003b643e7a RDI: ffffe8ffffc26938
  RBP: ffff8801ce447530 R08: 1ffff1003b643e6c R09: ffffe8ffffc26964
  R10: 0000000000000002 R11: fffff91ffff84d2d R12: ffffe8ffffc1f890
  R13: ffffe8ffffc26930 R14: ffffffff85cad9e0 R15: 0000000000000000
  FS:  0000000002641880(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 000000000043e670 CR3: 00000001d1d7a000 CR4: 00000000001406f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
    trace_block_getrq include/trace/events/block.h:423 [inline]
    __get_request block/blk-core.c:1283 [inline]
    get_request+0x1518/0x23b0 block/blk-core.c:1355
    blk_old_get_request block/blk-core.c:1402 [inline]
    blk_get_request+0x1d8/0x3c0 block/blk-core.c:1427
    sg_scsi_ioctl+0x117/0x750 block/scsi_ioctl.c:451
    sg_ioctl+0x192d/0x2ed0 drivers/scsi/sg.c:1070
    vfs_ioctl fs/ioctl.c:45 [inline]
    do_vfs_ioctl+0x1b1/0x1530 fs/ioctl.c:685
    SYSC_ioctl fs/ioctl.c:700 [inline]
    SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
    entry_SYSCALL_64_fastpath+0x1f/0xbe

block_get_rq::TP_fast_assign() has multiple redundant ->dev assignments.
Only one of them is NULL tolerant.  Favor the NULL tolerant one.

Fixes: 74d46992 ("block: replace bi_bdev with a gendisk pointer and partitions index")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>