Where 'current' is used to index over vm_map entries, use
'entry'. Where 'entry' is used to identify the starting point for
iteration, use 'first_entry'. These are the naming conventions used in
most of the vm_map.c code. Where VM_MAP_ENTRY_FOREACH can be used, do
so. Squeeze a few lines to fit in 80 columns. Where lines are being
modified for these reasons, look to remove style(9) violations.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D22458