lib/crypto/poly1305: Fix arm64's poly1305_blocks_arch()

For some reason arm64's Poly1305 code got changed to ignore the padbit
argument.  As a result, the output is incorrect when the message length
is not a multiple of 16 (which is not reached with the standard
ChaCha20Poly1305, but bcachefs could reach this).  Fix this.

Fixes: a59e5468 ("crypto: arm64/poly1305 - Add block-only interface")
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Tested-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lore.kernel.org/r/20250616010654.367302-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>