[GFS2] Don't flush everything on fdatasync

The gfs2_fsync() function was doing a journal flush on each
and every call. While this is correct, its also a lot of
overhead. This patch means that on fdatasync flushes we
rely on the VFS to flush the data for us and we don't do
a journal flush unless we really need to.

We have to do a journal flush for stuffed files though because
they have the data and the inode metadata in the same block.
Journaled files also need a journal flush too of course.

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