Define a vm_map method for user-space for advancing from a map entry
to its successor in cases where examining a map entry requires a
helper like kvm_read_all. Use that method, with kvm_read_all, to fix
procstat_getfiles_kvm, which tries to find the successor now without
using such a helper. This addresses a problem introduced by r355491.

Reviewed by: markj (previous version)
Discussed with: kib
Differential Revision: https://reviews.freebsd.org/D22728