fs/adfs: newdir: split out directory commit from update

After changing a directory, we need to update the sequence numbers and
calculate the new check byte before the directory is scheduled to be
written back to the media.  Since this needs to happen for any change
to the directory, move this into a separate method.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>