ceph: checking for IS_ERR instead of NULL

ceph_osdc_alloc_request() returns NULL on error, it never returns error
pointers.

Fixes: 5be0389d

 ('ceph: re-send AIO write request when getting -EOLDSNAP error')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>