crypto: rsa-pkcs1pad - Deduplicate set_{pub,priv}_key callbacks

pkcs1pad_set_pub_key() and pkcs1pad_set_priv_key() are almost identical.

The upcoming migration of sign/verify operations from rsa-pkcs1pad.c
into a separate crypto_template will require another copy of the exact
same functions.  When RSASSA-PSS and RSAES-OAEP are introduced, each
will need yet another copy.

Deduplicate the functions into a single one which lives in a common
header file for reuse by RSASSA-PKCS1-v1_5, RSASSA-PSS and RSAES-OAEP.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>