bcachefs: Switch to key_type_user, not logon

bcachefs: Switch to key_type_user, not logon

The only difference key_type_logon and key_type_user is that
key_type_logon keys can't be read by userspace.

However, userspace has actually been adding keys to both the logon and
user keychains, because userspace fsck requires the keychain interface -
so we might as well just use user and drop the logon keychain.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>