[GFS2] Alter direct I/O path

As per comments received, alter the GFS2 direct I/O path so that
it uses the standard read functions "out of the box". Needs a
small change to one of the VFS functions. This reduces the size
of the code quite a lot and also removes the need for one new export.

Some more work remains to be done, but this is the bones of the
thing.

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