[GFS2] Introduce gfs2_set_aops()

Just like ext3 we now have three sets of address space operations
to cover the cases of writeback, ordered and journalled data
writes. This means that the individual operations can now become
less complicated as we are able to remove some of the tests for
file data mode from the code.

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