vm_map_protect(): add VM_MAP_PROTECT_GROWSDOWN flag

which requests to propagate lowest stack segment protection to the grow gap.
This seems to be required for Linux emulation.

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