[GFS2] Add generation number

This adds a generation number for the eventual use of NFS to the
ondisk inode. Its backward compatible with the current code since
it doesn't really matter what the generation number is to start with,
and indeed since its set to zero, due to it being taken from padding
in both the inode and rgrp header, it should be fine.

The eventual plan is to use this rather than no_formal_ino in the
NFS filehandles. At that point no_formal_ino will be unused.

At the same time we also add a releasepages call back to the
"normal" address space for gfs2 inodes. Also I've removed a
one-linrer function thats not required any more.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>