io_uring: fix memory leak of uid in files registration

When there are no files for __io_sqe_files_scm() to process in the
range, it'll free everything and return. However, it forgets to put uid.

Fixes: 08a4517 ("io_uring: allow sparse fixed file sets")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/accee442376f33ce8aaebb099d04967533efde92.1648226048.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

fs/io_uring.c

@@ -8845,6 +8845,7 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
 			fput(fpl->fp[i]);
 	} else {
 		kfree_skb(skb);
+		free_uid(fpl->user);
 		kfree(fpl);
 	}