ocfs2: release the buffer head in ocfs2_do_truncate.

In ocfs2_do_truncate, we forget to release last_eb_bh which
will cause memleak. So call brelse in the end.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>