libmount: ignore name=value if only 'name' is expected

For example

	mount /srv/www /mnt -o rw,group=woven,dev,suid

the group= should not be interpreted as userspace mount option, because
umount(8) expects 'group' (without =<value>).

Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Karel Zak <kzak@redhat.com>