Adopt the deferring of object deallocation for the deleted map entries

Adopt the deferring of object deallocation for the deleted map entries
on map unlock to the lock downgrade and later read unlock operation.

System map entries cannot be backed by OBJT_VNODE objects, no need to
defer deallocation for them. Map entries from user maps do not require
the owner map for deallocation, and can be accumulated in the
thread-local list for freeing when a user map is unlocked.

Move the collection of entries for deferred reclamation into
vm_map_delete(). Create helper vm_map_process_deferred(), that is
called from locations where processing is feasible. Do not process
deferred entries in vm_map_unlock_and_wait() since map_sleep_mtx is
held.

Reviewed by:	alc, rstone (previous versions)
Tested by:	pho
MFC after:	2 weeks