9p: potential NULL dereference

[ Upstream commit 72ea0321 ]

p9_tag_alloc() is supposed to return error pointers, but we accidentally
return a NULL here.  It would cause a NULL dereference in the caller.

Link: http://lkml.kernel.org/m/20180926103934.GA14535@mwanda
Fixes: 996d5b4d ("9p: Use a slab for allocating requests")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/9p/client.c

@@ -287,7 +287,7 @@ p9_tag_alloc(struct p9_client *c, int8_t type, unsigned int max_size)
 	int tag;
 
 	if (!req)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	if (p9_fcall_init(c, &req->tc, alloc_msize))
 		goto free_req;