xfrm: Use xfrm_state selector for BEET input

For BEET the inner address and therefore family is stored in the
xfrm_state selector.  Use that when decapsulating an input packet
instead of incorrectly relying on a non-existent tunnel protocol.

Fixes: 5f24f41e ("xfrm: Remove inner/outer modes from input path")
Reported-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>