linuxolator: implement Linux' PROT_GROWSDOWN

From the Linux man page for mprotect(2):

PROT_GROWSDOWN
    Apply  the  protection  mode  down to the beginning of a mapping
    that grows downward (which should be a stack segment or a
    segment mapped with the MAP_GROWSDOWN flag set).

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