vm_phys_avail_check(): Check index parity, fix panic messages

The passed index must be the start of a chunk in phys_avail[], so must
be even. Test for that and print a separate panic message.

While here, fix panic messages: In one, the wrong chunk boundary was
printed, and in another, the desired but not the actual condition was
printed, possibly leading to confusion.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48626

(cherry picked from commit 125ef4e041fed40fed2d00b0ddd90fa0eb7b6ac3)