Add support for reference counting using C11 atomics

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

GH: #1500

crypto/dsa/dsa_lib.c

diff --git a/crypto/dsa/dsa_lib.c b/crypto/dsa/dsa_lib.c
index 42324c7..e24c6b5 100644
--- a/crypto/dsa/dsa_lib.c
+++ b/crypto/dsa/dsa_lib.c
@@ -120,7 +120,7 @@
     if (r == NULL)
         return;
 
-    CRYPTO_atomic_add(&r->references, -1, &i, r->lock);
+    CRYPTO_DOWN_REF(&r->references, &i, r->lock);
     REF_PRINT_COUNT("DSA", r);
     if (i > 0)
         return;
@@ -148,7 +148,7 @@
 {
     int i;
 
-    if (CRYPTO_atomic_add(&r->references, 1, &i, r->lock) <= 0)
+    if (CRYPTO_UP_REF(&r->references, &i, r->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("DSA", r);

crypto/dsa/dsa_locl.h

diff --git a/crypto/dsa/dsa_locl.h b/crypto/dsa/dsa_locl.h
index 9021fce..f575195 100644
--- a/crypto/dsa/dsa_locl.h
+++ b/crypto/dsa/dsa_locl.h
@@ -8,6 +8,7 @@
  */
 
 #include <openssl/dsa.h>
+#include "internal/refcount.h"
 
 struct dsa_st {
     /*
@@ -24,7 +25,7 @@
     int flags;
     /* Normally used to cache montgomery values */
     BN_MONT_CTX *method_mont_p;
-    int references;
+    CRYPTO_REF_COUNT references;
     CRYPTO_EX_DATA ex_data;
     const DSA_METHOD *meth;
     /* functional reference if 'meth' is ENGINE-provided */