ceph: fix dereference of null pointer cf

Currently in the case where kmem_cache_alloc fails the null pointer
cf is dereferenced when assigning cf->is_capsnap = false. Fix this
by adding a null pointer check and return path.

Cc: stable@vger.kernel.org
Addresses-Coverity: ("Dereference null return")
Fixes: b2f9fa1f ("ceph: correctly handle releasing an embedded cap flush")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1736,6 +1736,9 @@ struct ceph_cap_flush *ceph_alloc_cap_flush(void)
 	struct ceph_cap_flush *cf;
 
 	cf = kmem_cache_alloc(ceph_cap_flush_cachep, GFP_KERNEL);
+	if (!cf)
+		return NULL;
+
 	cf->is_capsnap = false;
 	return cf;
 }