[IPSEC]: Use dst->header_len when resizing on output

Currently we use x->props.header_len when resizing on output.
However, if we're resizing at all we might as well go the whole hog
and do it for the whole dst.

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