ocfs2: Kill the last naked wait_on_buffer() for cached reads.

ocfs2's cached buffer I/O goes through ocfs2_read_block(s)().  dir.c had
a naked wait_on_buffer() to wait for some readahead, but it should
use ocfs2_read_block() instead.

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