xfrm: get rid of another incorrect WARN

During fuzzing I regularly run into this WARN(). According to Herbert Xu,
this "certainly shouldn't be a WARN, it probably shouldn't print anything
either".

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>