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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>