X.509: fix comparisons of ->pkey_algo

X.509: fix comparisons of ->pkey_algo

->pkey_algo used to be an enum, but was changed to a string by commit
691b7a6f5497 ("X.509: Make algo identifiers text instead of enum").  But
two comparisons were not updated.  Fix them to use strcmp().

This bug broke signature verification in certain configurations,
depending on whether the string constants were deduplicated or not.

Fixes: 691b7a6f5497 ("X.509: Make algo identifiers text instead of enum")
Cc: <stable@vger.kernel.org> # v4.6+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>