Fix panic on initialize of "vm reserv" per-superpage lock in case when RAM ends at upper boundary of address space.
Observed on ARM32 board BPI-R2 (2GB RAM 0x80000000-0xffffffff).

PR:		235362
Reviewed by:	kib, markj, alc
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D21272