crypto: x86/cast6 - Fix RBP usage

Using RBP as a temporary register breaks frame pointer convention and
breaks stack traces when unwinding from an interrupt in the crypto code.

Use R15 instead of RBP.  R15 can't be used as the RID1 register because
of x86 instruction encoding limitations.  So use R15 for CTX and RDI for
CTX.  This means that CTX is no longer an implicit function argument.
Instead it needs to be explicitly copied from RDI.

Reported-by: Eric Biggers <ebiggers@google.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Eric Biggers <ebiggers@google.com>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>