Destroy DMAPI session when main smbd daemon exits.

Destroy DMAPI session when main smbd daemon exits.

DMAPI session is precious resource maintained at kernel level. We open one of them and use across multiple smbd daemons
but once last of them exits, DMAPI session needs to be destroyed. There are some HSM implementations which fail to
shutdown when opened DMAPI sessions left. Ensure we shutdown our session when it is really not needed anymore.
This is what recommended by DMAPI specification anyway.