Use available memory statistic when possible

Update function returning size of free memory to use available statistic
if that's supported by given kernel. This function is used to check
whether there is enough memory to start cache, and available statistic is
preferred for this check over free statistic, as it does include size of
page cache allocations which can be easily reclaimed by the system.

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

modules/cas_cache/ocf_env.h

@@ -1,5 +1,5 @@
 /*
-* Copyright(c) 2012-2021 Intel Corporation
+* Copyright(c) 2012-2022 Intel Corporation
 * SPDX-License-Identifier: BSD-3-Clause
 */
 
@@ -22,7 +22,7 @@
 
 static inline uint64_t env_get_free_memory(void)
 {
-	return cas_global_zone_page_state(NR_FREE_PAGES) << PAGE_SHIFT;
+	return cas_get_free_memory();
 }
 
 static inline void *env_malloc(size_t size, int flags)