crypto: skcipher - remove support for physical address walks

Since the physical address support in skcipher_walk is not used anymore,
remove all the code associated with it.  This includes:

- The skcipher_walk_async() and skcipher_walk_complete() functions;

- The SKCIPHER_WALK_PHYS flag and everything conditional on it;

- The buffers, phys, and virt.page fields in struct skcipher_walk;

- struct skcipher_walk_buffer.

As a result, skcipher_walk now just supports virtual addresses.
Physical address support in skcipher_walk is unneeded because drivers
that need physical addresses just use the scatterlists directly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>