sg: Fix double-free when drives detach during SG_IO

In sg_common_write(), we free the block request and return -ENODEV if
the device is detached in the middle of the SG_IO ioctl().

Unfortunately, sg_finish_rem_req() also tries to free srp->rq, so we
end up freeing rq->cmd in the already free rq object, and then free
the object itself out from under the current user.

This ends up corrupting random memory via the list_head on the rq
object. The most common crash trace I saw is this:

  ------------[ cut here ]------------
  kernel BUG at block/blk-core.c:1420!
  Call Trace:
  [<ffffffff81281eab>] blk_put_request+0x5b/0x80
  [<ffffffffa0069e5b>] sg_finish_rem_req+0x6b/0x120 [sg]
  [<ffffffffa006bcb9>] sg_common_write.isra.14+0x459/0x5a0 [sg]
  [<ffffffff8125b328>] ? selinux_file_alloc_security+0x48/0x70
  [<ffffffffa006bf95>] sg_new_write.isra.17+0x195/0x2d0 [sg]
  [<ffffffffa006cef4>] sg_ioctl+0x644/0xdb0 [sg]
  [<ffffffff81170f80>] do_vfs_ioctl+0x90/0x520
  [<ffffffff81258967>] ? file_has_perm+0x97/0xb0
  [<ffffffff811714a1>] SyS_ioctl+0x91/0xb0
  [<ffffffff81602afb>] tracesys+0xdd/0xe2
    RIP [<ffffffff81281e04>] __blk_put_request+0x154/0x1a0

The solution is straightforward: just set srp->rq to NULL in the
failure branch so that sg_finish_rem_req() doesn't attempt to re-free
it.

Additionally, since sg_rq_end_io() will never be called on the object
when this happens, we need to free memory backing ->cmd if it isn't
embedded in the object itself.

KASAN was extremely helpful in finding the root cause of this bug.

Signed-off-by: Calvin Owens <calvinowens@fb.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
