ocfs2: Don't repeat ocfs2_xattr_block_find()

ocfs2_xattr_block_get() looks up the xattr in a startlingly familiar
way; it's identical to the function ocfs2_xattr_block_find().  Let's just
use the later in the former.

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