me further work will probably be needed to set a suitable value
for DIO as well, but for now thats left at the maximum 512 (see
ops_address.c:gfs2_get_block_direct).

There is probably a lot more that can be done to improve bmap for GFS2,
but this is a good first step.

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