ocfs2: Respond to on-disk corruption in the extent map code.

The extent map code has long noticed when the on-disk extent information
is corrupt.  However, so far it has only returned an error.  We should
take the filesystem read-only, as it is corrupt.

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