[IPSEC] esp: Remove NAT-T checksum invalidation for BEET

I pointed this out back when this patch was first proposed but it looks like
it got lost along the way.

The checksum only needs to be ignored for NAT-T in transport mode where
we lose the original inner addresses due to NAT.  With BEET the inner
addresses will be intact so the checksum remains valid.

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