ecryptfs: use kasprintf in ecryptfs_crypto_api_algify_cipher_name

Use kasprintf() to simplify ecryptfs_crypto_api_algify_cipher_name().

Use const char * for the read-only cipher name and chaining modifier
while at it.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Tyler Hicks <code@tyhicks.com>