[CRYPTO] skcipher: Add crypto_grab_skcipher interface

Note: From now on the collective of ablkcipher/blkcipher/givcipher will
be known as skcipher, i.e., symmetric key cipher.  The name blkcipher has
always been much of a misnomer since it supports stream ciphers too.

This patch adds the function crypto_grab_skcipher as a new way of getting
an ablkcipher spawn.  The problem is that previously we did this in two
steps, first getting the algorithm and then calling crypto_init_spawn.

This meant that each spawn user had to be aware of what type and mask to
use for these two steps.  This is difficult and also presents a problem
when the type/mask changes as they're about to be for IV generators.

The new interface does both steps together just like crypto_alloc_ablkcipher.

As a side-effect this also allows us to be stronger on type enforcement
for spawns.  For now this is only done for ablkcipher but it's trivial
to extend for other types.

This patch also moves the type/mask logic for skcipher into the helpers
crypto_skcipher_type and crypto_skcipher_mask.

Finally this patch introduces the function crypto_require_sync to determine
whether the user is specifically requesting a sync algorithm.

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

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