crypto: xts - prefix function and struct names with "xts"

Overly-generic names can cause problems like naming collisions,
confusing crash reports, and reduced grep-ability.  E.g. see
commit d099ea6e ("crypto - Avoid free() namespace collision").

Clean this up for the xts template by prefixing the names with "xts_".

(I didn't use "crypto_xts_" instead because that seems overkill.)

Also constify the tfm context in a couple places, and make
xts_free_instance() use the instance context structure so that it
doesn't just assume the crypto_skcipher_spawn is at the beginning.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>