kernel - TMPFS - Features, don't sync on umount, enforce snocache on root

kernel - TMPFS - Features, don't sync on umount, enforce snocache on root

* The umount code was syncing the contents of tmpfs to swap.  Fix it so it
  doesn't.

* Set the snocache flag on the tmpfs root node and silently enforce it on
  any chflags operation.  This prevents swapcache from trying to pageout
  file data from tmpfs (which is already backed by swap).