vm_fault: move FAULT_* return codes out of range for Mach errors

This way a possible clash between FAULT_* and KERN_* numbering is
avoided, and panics checks for fault_status confusion become more
efficient.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D40771