[GFS2] Remove unused constants

Three of the DIO constants were not being used, so remove them.

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

diff --cc fs/gfs2/incore.h
Simple merge

diff --cc fs/gfs2/meta_io.c
Simple merge

diff --cc fs/gfs2/ops_address.c
Simple merge