crypto: user - Fix crypto_alg_match race

crypto: user - Fix crypto_alg_match race

commit 016baaa1183bb0c5fb2a7de42413bba8a51c1bc8 upstream.

The function crypto_alg_match returns an algorithm without taking
any references on it.  This means that the algorithm can be freed
at any time, therefore all users of crypto_alg_match are buggy.

This patch fixes this by taking a reference count on the algorithm
to prevent such races.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>