libmount: don't remove user= when executed by root

libmount: don't remove user= when executed by root

The original mount(8) allows to store arbitrary user= option to mtab
file if called by root user. For example:

  # mount -f foo /bar -t xxx -o rw,user=kzak

the new mount removes the 'user=' and 'users' options at all for root
user. This is regression. The original functionality is necessary by
'sshfs' where fuse writes to mtab file by mount(8).

Reported-by: Juergen Daubert <jue@jue.li> (and 'horrorStruck' on IRC)
Signed-off-by: Karel Zak <kzak@redhat.com>