tmpfs: add nomtime mount option,

which disables tracking mtime updates due to writes through the shared
mapped areas backed by tmpfs files. This removes periodic scans which
downgrades rw mapped pages to ro to note the writes.

Suggested by: mjg
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23432