crypto: sig - Rename crypto_sig_maxsize() to crypto_sig_keysize()

crypto_sig_maxsize() is a bit of a misnomer as it doesn't return the
maximum signature size, but rather the key size.

Rename it as well as all implementations of the ->max_size callback.
A subsequent commit introduces a crypto_sig_maxsize() function which
returns the actual maximum signature size.

While at it, change the return type of crypto_sig_keysize() from int to
unsigned int for consistency with crypto_akcipher_maxsize().  None of
the callers checks for a negative return value and an error condition
can always be indicated by returning zero.

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