ext3: Flush disk caches on fsync when needed

In case we fsync() a file and inode is not dirty, we don't force a transaction
to disk and hence don't flush disk caches. Thus file data could be just in disk
caches and not on persistent storage. Fix the problem by flushing disk caches
if we didn't force a transaction commit.

Signed-off-by: Jan Kara <jack@suse.cz>