[XFRM]: Expand XFRM_MAX_DEPTH for route optimization.

XFRM_MAX_DEPTH is a limit of transformation states to be applied to the same
flow. Two more extension headers are used by Mobile IPv6 transformation.
Based on MIPL2 kernel patch.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>