[syzbot] memory leak in keyctl_watch_key

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+6e2de48f06cdb2884bfc at syzkaller.appspotmail.com

Tested on:

commit:         c82efd1d watch_queue: Actually free the watch
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git
kernel config:  https://syzkaller.appspot.com/x/.config?x=7683c993c23430bc
dashboard link: https://syzkaller.appspot.com/bug?extid=6e2de48f06cdb2884bfc
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.