linux-phytec-ti

virtio-blk: fix NULL checking in virtblk_alloc_req()

Smatch complains about the inconsistent NULL checking here.  Fix it to
return NULL on failure.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (fixed accidental deletion)