Allow implicit lock conversion for pre/post callbacks.

Allow implicit lock conversion for pre/post callbacks.

This is unnoticed regression from commit 31672ff60e405795cad70d6d7888ac011f5373ce

The pre/post callback need to convert lock always, local node
is going to modify metadata in this case, it it fails conversion,
the call is ignored.

Also it fixes bug when the lock is not yet held, we cannot set LKF_CONVERT
in this case, it will fail because this lock do not exist.

Note that the automatic conversion is still disabled in activate
call, so the original fix (reactivation of exlusive LV) should
be still in place.