ext2_sync_file() forgets to clear the inode's dirty bits, so we write the
inode on every fsync(), even if it hasn't changed.

Fix that up via the new sync_file() API which correctly manages the inode
state bits and the superblock inode lists.

When performing file overwrite on IDE with and without writeback caching
enabled this patch approximately doubles fsync() speed, bringing it into line
with O_SYNC writes.

Also, fix up the return value handling in ext2_sync_file().

Credit due to Jeffrey Siegal <jbs@quiotix.com> who noticed the performance
discrepancy and wrote a test app.