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