mm/madvise: fix memory leak from process_madvise

The early return in process_madvise() will produce a memory leak.

Fix it.

Fixes: ecb8ac8 ("mm/madvise: introduce process_madvise() syscall: an external memory hinting API")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20201116155132.GA3805951@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: edumazet

mm/madvise.c

@@ -1231,8 +1231,6 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
 		ret = total_len - iov_iter_count(&iter);
 
 	mmput(mm);
-	return ret;
-
 release_task:
 	put_task_struct(task);
 put_pid: