gfs2: set FMODE_CAN_ODIRECT instead of a dummy direct_IO method

Since commit a2ad63da ("VFS: add FMODE_CAN_ODIRECT file flag"), file
systems can just set the FMODE_CAN_ODIRECT flag at open time instead of
wiring up a dummy direct_IO method to indicate support for direct I/O.

Remove .direct_IO from gfs2_aops and set FMODE_CAN_ODIRECT in
gfs2_open_common for regular files that do not use data journalling.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>