The vmtotal sysctl handler marks active vm objects to calculate

The vmtotal sysctl handler marks active vm objects to calculate
statistics.  Marking is done by setting the OBJ_ACTIVE flag.  The
flags change is locked, but the problem is that many parts of system
assume that vm object initialization ensures that no other code could
change the object, and thus performed lockless.  The end result is
corrupted flags in vm objects, most visible is spurious OBJ_DEAD flag,
causing random hangs.

Avoid the active object marking, instead provide equally inexact but
immutable is_object_alive() definition for the object mapped state.

Avoid iterating over the processes mappings altogether by using
arguably improved definition of the paging thread as one which sleeps
on the v_free_count.

PR:	204764
Diagnosed by:	pho
Tested by:	pho (previous version)
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (gjb)