crypto: af_alg - use list_for_each_entry() in af_alg_count_tsgl()

af_alg_count_tsgl() iterates through a list without modifying it, so use
list_for_each_entry() rather than list_for_each_entry_safe().  Also make
the pointers 'const' to make it clearer that nothing is modified.

No actual change in behavior.

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