libmount: always ignore user=<name>

libmount: always ignore user=<name>

The library (on mount) cares about "user", but has to ignore
"user=name". It works as expected, but only for non-root users. We
need to ignore it also root to be compatible with /sbin/mount.cifs
where some people still use "user=" (rather than "username=").

References: fe0b1e793c9017edba72768e2e0b4c769c204604
Addresses: https://github.com/util-linux/util-linux/issues/2315
Signed-off-by: Karel Zak <kzak@redhat.com>