fscrypt: use crypto_skcipher_driver_name()

Crypto API users shouldn't really be accessing struct skcipher_alg
directly.  <crypto/skcipher.h> already has a function
crypto_skcipher_driver_name(), so use that instead.

No change in behavior.

Link: https://lore.kernel.org/r/20191209203810.225302-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>