crypto: sig - Introduce sig_alg backend

Commit 6cb8815f41 ("crypto: sig - Add interface for sign/verify")
began a transition of asymmetric sign/verify operations from
crypto_akcipher to a new crypto_sig frontend.

Internally, the crypto_sig frontend still uses akcipher_alg as backend,
however:

   "The link between sig and akcipher is meant to be temporary.  The
    plan is to create a new low-level API for sig and then migrate
    the signature code over to that from akcipher."
    https://lore.kernel.org/r/ZrG6w9wsb-iiLZIF@gondor.apana.org.au/

   "having a separate alg for sig is definitely where we want to
    be since there is very little that the two types actually share."
    https://lore.kernel.org/r/ZrHlpz4qnre0zWJO@gondor.apana.org.au/

Take the next step of that migration and augment the crypto_sig frontend
with a sig_alg backend to which all algorithms can be moved.

During the migration, there will briefly be signature algorithms that
are still based on crypto_akcipher, whilst others are already based on
crypto_sig.  Allow for that by building a fork into crypto_sig_*() API
calls (i.e. crypto_sig_maxsize() and friends) such that one of the two
backends is selected based on the transform's cra_type.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/uapi/linux/cryptouser.h

@@ -64,6 +64,7 @@ enum crypto_attr_type_t {
 	CRYPTOCFGA_STAT_AKCIPHER,	/* No longer supported, do not use. */
 	CRYPTOCFGA_STAT_KPP,		/* No longer supported, do not use. */
 	CRYPTOCFGA_STAT_ACOMP,		/* No longer supported, do not use. */
+	CRYPTOCFGA_REPORT_SIG,		/* struct crypto_report_sig */
 	__CRYPTOCFGA_MAX
 
 #define CRYPTOCFGA_MAX (__CRYPTOCFGA_MAX - 1)
@@ -207,6 +208,10 @@ struct crypto_report_acomp {
 	char type[CRYPTO_MAX_NAME];
 };
 
+struct crypto_report_sig {
+	char type[CRYPTO_MAX_NAME];
+};
+
 #define CRYPTO_REPORT_MAXSIZE (sizeof(struct crypto_user_alg) + \
 			       sizeof(struct crypto_report_blkcipher))