kernel - Do not clean VM pages on fsync() for tmpfs

kernel - Do not clean VM pages on fsync() for tmpfs

* tmpfs fsync()s are basically NOPs, but the kernel had some
  code to clean the VM object (flush to backing store) on
  fsync, which would actually cause pages to go out to swap
  unnecessarily.

* Detect the appropriate flag and turn this off.  Only effects
  tmpfs.

* Improves poudriere and other tmpfs-related use cases.