mmap(MAP_STACK): on stack grow, use original protection

If mprotect(2) changed protection in the bottom of the currently grown
stack region, currently the changed protection would be used for the
stack grow on next fault. This is arguably unexpected.

Store the original protection for the entry at mmap(2) time in the
offset member of the gap vm_map_entry, and use it for protection of the
grown stack region.

PR: 272585
Reported by: John F. Carr <jfc@mit.edu>
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41089