Simplify the check for non-dumpable VM object types

Simplify the check for non-dumpable VM object types

OBJT_DEFAULT, _SWAP, _VNODE and _PHYS is exactly the set of
non-fictitious object types, so just check for OBJ_FICTITIOUS.  The
check no longer excludes dead objects, but such objects have to be
handled regardless.

No functional change intended.

Reviewed by:	alc, dougm, kib
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26589