ocfs2: fdatasync should skip unimportant metadata writeout

In ocfs2, fdatasync and fsync are identical.
I think fdatasync should skip committing transaction when
inode->i_state is set just I_DIRTY_SYNC and this indicates
only atime or/and mtime updates.
Following patch improves fdatasync throughput.

#sysbench --num-threads=16 --max-requests=300000 --test=fileio
--file-block-size=4K --file-total-size=16G --file-test-mode=rndwr
--file-fsync-mode=fdatasync run

Results:
-2.6.30-rc8
Test execution summary:
    total time:                          107.1445s
    total number of events:              119559
    total time taken by event execution: 116.1050
    per-request statistics:
         min:                            0.0000s
         avg:                            0.0010s
         max:                            0.1220s
         approx.  95 percentile:         0.0016s

Threads fairness:
    events (avg/stddev):           7472.4375/303.60
    execution time (avg/stddev):   7.2566/0.64

-2.6.30-rc8-patched
Test execution summary:
    total time:                          86.8529s
    total number of events:              300016
    total time taken by event execution: 24.3077
    per-request statistics:
         min:                            0.0000s
         avg:                            0.0001s
         max:                            0.0336s
         approx.  95 percentile:         0.0001s

Threads fairness:
    events (avg/stddev):           18751.0000/718.75
    execution time (avg/stddev):   1.5192/0.05

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>