Register .remount_fs handler

Register .remount_fs handler

Register the missing .remount_fs handler.  This handler isn't strictly
required because the VFS does a pretty good job updating most of the
MS_* flags.  However, there's no harm in using the hook to call the
registered zpl callback for various MS_* flags.  Additionaly, this
allows us to lay the ground work for more complicated argument parsing
in the future.