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
492bf5f6366f ("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: 492bf5f6366f ("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>