crypto: algif - avoid excessive use of socket buffer in skcipher

On archs with PAGE_SIZE >= 64 KiB the function skcipher_alloc_sgl()
fails with -ENOMEM no matter what user space actually requested.
This is caused by the fact sock_kmalloc call inside the function tried
to allocate more memory than allowed by the default kernel socket buffer
size (kernel param net.core.optmem_max).

Signed-off-by: Ondrej Kozina <okozina@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>