pmap: Prefer consistent naming for loader tunable

The sysctl knob 'vm.pmap.pv_entry_max' becomes a loader tunable since
7ff48af7040f (Allow a specific setting for pv entries) but is fetched
from system environment 'vm.pmap.pv_entries'. That is inconsistent and
obscure.

This reverts 36e1b9702e21 (Correct the tunable name in the message).

PR: 231577
Reviewed by: jhibbits, alc, kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42274

(cherry picked from commit 02320f64209563e35fa371fc5eac94067f688f7f)
(cherry picked from commit e53f8ca323e8e563d4b55883fc3544bea75aab29)