ocfs2: Convert ocfs2_read_dir_block() to ocfs2_read_virt_blocks()

Now that we've centralized the ocfs2_read_virt_blocks() code, let's use
it in ocfs2_read_dir_block().

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>