procfs_map: switch to use vm_object_kvme_type

to get object type, and stop enumerating OBJT_XXX constants. This also
provides properly a pointer for the vnode, if object backs any.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30168