fsnotify: clean up fsnotify_prepare/finish_user_wait()

This patch doesn't actually fix any bug, just paves the way for fixing mark
and group pinning.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org> # v4.12
Signed-off-by: Jan Kara <jack@suse.cz>