crypto: polyval - Rename conflicting functions

crypto: polyval - Rename conflicting functions

Rename polyval_init() and polyval_update(), in preparation for adding
library functions with the same name to <crypto/polyval.h>.

Note that polyval-generic.c will be removed later, as it will be
superseded by the library.  This commit just keeps the kernel building
for the initial introduction of the library.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251109234726.638437-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>