crypto: akcipher - pass instance to crypto_grab_akcipher()

Initializing a crypto_akcipher_spawn currently requires:

1. Set spawn->base.inst to point to the instance.
2. Call crypto_grab_akcipher().

But there's no reason for these steps to be separate, and in fact this
unneeded complication has caused at least one bug, the one fixed by
commit 6db43410

 ("crypto: adiantum - initialize crypto_spawn::inst")

So just make crypto_grab_akcipher() take the instance as an argument.

To keep the function call from getting too unwieldy due to this extra
argument, also introduce a 'mask' variable into pkcs1pad_create().

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