[IPSEC]: Fix crypto_alloc_comp error checking

The function crypto_alloc_comp returns an errno instead of NULL
to indicate error.  So it needs to be tested with IS_ERR.

This is based on a patch by Vicen Beltran Querol.

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