In order to remove performance impact of having the extra u32 in every
single flowi, this change removes the flowi_xfrm struct, prefering to
take the if_id as a method parameter where needed.

In the inbound direction, if_id is only needed during the
__xfrm_check_policy() function, and the if_id can be determined at that
point based on the skb. As such, xfrmi_decode_session() is only called
with the skb in __xfrm_check_policy().

In the outbound direction, the only place where if_id is needed is the
xfrm_lookup() call in xfrmi_xmit2(). With this change, the if_id is
directly passed into the xfrm_lookup_with_ifid() call. All existing
callers can still call xfrm_lookup(), which uses a default if_id of 0.

This change does not change any behavior of XFRMIs except for improving
overall system performance via flowi size reduction.

This change has been tested against the Android Kernel Networking Tests:

https://android.googlesource.com/kernel/tests/+/master/net/testSigned-off-by: Benedict Wong <benedictwong@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>