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>

crypto/algif_skcipher.c

@@ -49,7 +49,7 @@ struct skcipher_ctx {
 	struct ablkcipher_request req;
 };
 
-#define MAX_SGL_ENTS ((PAGE_SIZE - sizeof(struct skcipher_sg_list)) / \
+#define MAX_SGL_ENTS ((4096 - sizeof(struct skcipher_sg_list)) / \
 		      sizeof(struct scatterlist) - 1)
 
 static inline int skcipher_sndbuf(struct sock *sk)