crypto: crypto_xor - use helpers for unaligned accesses

Dereferencing a misaligned pointer is undefined behavior in C, and may
result in codegen on architectures such as ARM that trigger alignments
traps and expensive fixups in software.

Instead, use the get_aligned()/put_aligned() accessors, which are cheap
or even completely free when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y.

In the converse case, the prior alignment checks ensure that the casts
are safe, and so no unaligned accessors are necessary.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>