vm_object: Widen the flags field

Take advantage of a nearby 2-byte hole to avoid growing the struct.
This way, only the offsets of "flags" and "pg_color" change. Bump
__FreeBSD_version since some out-of-tree kernel modules may access these
fields, though I haven't found any examples so far.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35905