ext2: Fold ext2_commit_super() into ext2_sync_super()

Both function originally did similar things except that ext2_sync_super()
is returning after the call to sync_dirty_buffer(sbh). Therefore this
patch adds a wait flag to tell ext2_sync_super() if it has to call
sync_dirty_buffer() to wait for in-progress I/O to finish.

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