Check paddr for overflow.
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