Use default kernel export mechanism when symbol lookup is not available

Since kernel 5.7 kallsyms_on_each_symbol() is not available.
NOTE: This affects ability to perform upgrade in flight on kernels 5.7+.

Signed-off-by: Robert Baldyga <robert.baldyga@intel.com>

modules/cas_cache/main.c

@@ -51,6 +51,8 @@ bool in_upgrade;
 ocf_ctx_t cas_ctx;
 struct casdsk_functions_mapper casdisk_functions;
 
+#ifdef SYMBOL_LOOKUP_SUPPORTED
+
 struct exported_symbol {
 	char *name;
 	unsigned long addr;
@@ -74,6 +76,16 @@ int static cas_find_symbol(void *data, const char *namebuf,
              return -EINVAL; \
 })
 
+#else
+
+#include "../cas_disk/cas_disk.h"
+#include "../cas_disk/exp_obj.h"
+#define cas_lookup_symbol(f) ({ \
+	casdisk_functions.f = (void *)f; \
+})
+
+#endif
+
 int static cas_casdisk_lookup_funtions(void)
 {
 	mutex_lock(&module_mutex);