crypto: riscv/sha512 - Stop depending on sha512_generic_block_fn

crypto: riscv/sha512 - Stop depending on sha512_generic_block_fn

sha512_generic_block_fn() will no longer be available when the SHA-512
support in the old-school crypto API is changed to just wrap the SHA-512
library.  Replace the use of sha512_generic_block_fn() in
sha512-riscv64-glue.c with temporary code that uses the library's
__sha512_update().  This is just a temporary workaround to keep the
kernel building and functional at each commit; this code gets superseded
when the RISC-V optimized SHA-512 is migrated to lib/crypto/ anyway.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160320.2888-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>