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/dso/dso_lib.c

diff --git a/crypto/dso/dso_lib.c b/crypto/dso/dso_lib.c
index 52816df..8f185b3 100644
--- a/crypto/dso/dso_lib.c
+++ b/crypto/dso/dso_lib.c
@@ -65,7 +65,7 @@
     if (dso == NULL)
         return (1);
 
-    if (CRYPTO_atomic_add(&dso->references, -1, &i, dso->lock) <= 0)
+    if (CRYPTO_DOWN_REF(&dso->references, &i, dso->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("DSO", dso);
@@ -107,7 +107,7 @@
         return 0;
     }
 
-    if (CRYPTO_atomic_add(&dso->references, 1, &i, dso->lock) <= 0)
+    if (CRYPTO_UP_REF(&dso->references, &i, dso->lock) <= 0)
         return 0;
 
     REF_PRINT_COUNT("DSO", r);

crypto/dso/dso_locl.h

diff --git a/crypto/dso/dso_locl.h b/crypto/dso/dso_locl.h
index fbfad05..14a0ccb 100644
--- a/crypto/dso/dso_locl.h
+++ b/crypto/dso/dso_locl.h
@@ -11,6 +11,7 @@
 #include "internal/cryptlib.h"
 #include "internal/dso.h"
 #include "internal/dso_conf.h"
+#include "internal/refcount.h"
 
 /**********************************************************************/
 /* The low-level handle type used to refer to a loaded shared library */
@@ -24,7 +25,7 @@
      * "Handles" and such go in a STACK.
      */
     STACK_OF(void) *meth_data;
-    int references;
+    CRYPTO_REF_COUNT references;
     int flags;
     /*
      * For use by applications etc ... use this for your bits'n'pieces, don't