vm_map_growstack(): handle max protection for stacks

Do not assume that protection is same as max_protection. Store both in
offset, packed in the same way as the prot syscall parameter.

Reviewed by: alc, markj (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41099