Add linux_madvise(2) instead of having Linux apps call the native
FreeBSD madvise(2) directly. While some of the flag values match,
most don't.

PR: kern/230160
Reported by: markj
Reviewed by: markj
Discussed with: brooks, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25272