Btrfs: fdatasync should skip metadata writeout

In btrfs, fdatasync and fsync are identical, but
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.

--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:                          1980.6540s
    total number of events:              10001
    total time taken by event execution: 1192.9804
    per-request statistics:
         min:                            0.0000s
         avg:                            0.1193s
         max:                            15.3720s
         approx.  95 percentile:         0.7257s

Threads fairness:
    events (avg/stddev):           625.0625/151.32
    execution time (avg/stddev):   74.5613/9.46

-2.6.30-rc8-patched
Test execution summary:
    total time:                          1695.9118s
    total number of events:              10000
    total time taken by event execution: 871.3214
    per-request statistics:
         min:                            0.0000s
         avg:                            0.0871s
         max:                            10.4644s
         approx.  95 percentile:         0.4787s

Threads fairness:
    events (avg/stddev):           625.0000/131.86
    execution time (avg/stddev):   54.4576/8.98
Signed-off-by: NHisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -1157,6 +1157,8 @@ int btrfs_sync_file(struct file *file, struct dentry *dentry, int datasync)
 	btrfs_wait_ordered_range(inode, 0, (u64)-1);
 	root->log_batch++;
 
+	if (datasync && !(inode->i_state & I_DIRTY_PAGES))
+		goto out;
 	/*
 	 * ok we haven't committed the transaction yet, lets do a commit
 	 */