xfrm: Remove xfrm_state_genid

The xfrm state genid only needs to be matched against the copy
saved in xfrm_dst.  So we don't need a global genid at all.  In
fact, we don't even need to initialise it.

Based on observation by Timo Teräs.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>