[XFRM]: beet: fix IP option encapsulation

Beet mode calculates an incorrect value for the transport header location
when IP options are present, resulting in encapsulation errors.

The correct location is 4 or 8 bytes before the end of the original IP
header, depending on whether the pseudo header is padded.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>