userdb: suppress lookups outside of indicated ranges

userdb: suppress lookups outside of indicated ranges

When doing a userdb lookup we might end up issuing a lot of IPC calls in
parallel to backends and cause them all to do work, with us waiting for
it. Let's optimize this a bit, and indicate on the socket inodes via
xattrs hints which kind of records are provided by a backend. That way
we can suppress lookups to them and optimize runtime behaviour.

This only works on Linux 7.1 and newer where socket inodes gained
support for extended attributes.