ocfs2: Wrap dirblock reads in a dedicated function.

We have ocfs2_bread() as a vestige of the original ext-based dir code.
It's only used by directories, though.  Turn it into
ocfs2_read_dir_block(), with a prototype matching the other metadata
read functions.  It's set up to validate dirblocks when the time comes.

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