crypto: x86/salsa20 - cleanup and convert to skcipher API

Convert salsa20-asm from the deprecated "blkcipher" API to the
"skcipher" API, in the process fixing it up to use the generic helpers.
This allows removing the salsa20_keysetup() and salsa20_ivsetup()
assembly functions, which aren't performance critical; the C versions do
just fine.

This also fixes the same bug that salsa20-generic had, where the state
array was being maintained directly in the transform context rather than
on the stack or in the request context.  Thus, if multiple threads used
the same Salsa20 transform concurrently they produced the wrong results.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/Kconfig

@@ -1339,6 +1339,7 @@ config CRYPTO_SALSA20_586
 	tristate "Salsa20 stream cipher algorithm (i586)"
 	depends on (X86 || UML_X86) && !64BIT
 	select CRYPTO_BLKCIPHER
+	select CRYPTO_SALSA20
 	help
 	  Salsa20 stream cipher algorithm.
 
@@ -1352,6 +1353,7 @@ config CRYPTO_SALSA20_X86_64
 	tristate "Salsa20 stream cipher algorithm (x86_64)"
 	depends on (X86 || UML_X86) && 64BIT
 	select CRYPTO_BLKCIPHER
+	select CRYPTO_SALSA20
 	help
 	  Salsa20 stream cipher algorithm.