libmount: cleanup locking in table update code

libmount: cleanup locking in table update code

* use reference counting for libmnt_lock

* remove lock use from mnt_update_already_done(), it's read-only
  operation and utab update is atomic (based on rename(2))

* keep the lock instance mandatory for all low-level update_*
  functions. The lock is always initialized, so all the 'if(lc)'
  are unnecessary.

Signed-off-by: Karel Zak <kzak@redhat.com>