crypto: skcipher - optimize initializing skcipher_walk fields

The helper functions like crypto_skcipher_blocksize() take in a pointer
to a tfm object, but they actually return properties of the algorithm.
As the Linux kernel is compiled with -fno-strict-aliasing, the compiler
has to assume that the writes to struct skcipher_walk could clobber the
tfm's pointer to its algorithm.  Thus it gets repeatedly reloaded in the
generated code.  Therefore, replace the use of these helper functions
with staightforward accesses to the struct fields.

Note that while *users* of the skcipher and aead APIs are supposed to
use the helper functions, this particular code is part of the API
*implementation* in crypto/skcipher.c, which already accesses the
algorithm struct directly in many cases.  So there is no reason to
prefer the helper functions here.

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