fscrypt: Replace mk_users keyring with simple list

fscrypt: Replace mk_users keyring with simple list

Change mk_users (the set of user claims to an fscrypt master key) from a
'struct key' keyring to a simple linked list.

It's still a collection of 'struct key' for quota tracking.  It was
originally thought to be natural that a collection of 'struct key'
should be held in a 'struct key' keyring.  In reality, it's just been
causing problems, similar to how using 'struct key' for the filesystem
keyring caused problems and was removed in commit d7e7b9af104c
("fscrypt: stop using keyrings subsystem for fscrypt_master_key").

Commit d3a7bd420076 ("fscrypt: clear keyring before calling key_put()")
fixed mk_users cleanup to be synchronous.  But that apparently wasn't
enough: the keyring subsystem's redundant locking is still generating
lockdep false positives due to the interaction with filesystem reclaim.

With the simple list, the redundant locking and lockdep issue goes away.

Of course, searching a linked list is linear-time whereas the
'struct key' keyring used a fancy constant-time associative array.  But
that's fine here, since in practice there's just one entry in the list.
In fact the new code is much faster in practice, since it's much smaller
and doesn't have to convert the kuid_t into a string to search for it.

Reported-by: syzbot+f55b043dacf43776b50c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f55b043dacf43776b50c
Reported-by: Mohammed EL Kadiri <med08elkadiri@gmail.com>
Closes: https://lore.kernel.org/keyrings/20260614150041.21172-1-med08elkadiri@gmail.com/
Fixes: 23c688b54016 ("fscrypt: allow unprivileged users to add/remove keys for v2 policies")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260618221921.87896-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>