Reuse kern_proc_vmmap_resident() for procfs_map resident count.

The existing algorithm in procfs_map() to calculate count of resident
pages in an entry is too primitive, resulting in too long run time for
large sparse mapping entries. Re-use the kern_proc_vmmap_resident()
from kern_proc.c which only looks at the existing pages in the
iterations.

Also, this makes procfs to honor kern.proc_vmmap_skip_resident_count,
if user does not need this information.

Reported by: Glenn Weinberg <glenn.weinberg@intel.com>
PR: 224532
No objections from: des (procfs maintainer)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13595