mm: compaction: fix compiler warning when CONFIG_COMPACTION=n

The below warning is reported when CONFIG_COMPACTION=n:

   mm/compaction.c:56:27: warning: 'HPAGE_FRAG_CHECK_INTERVAL_MSEC' defined but not used [-Wunused-const-variable=]
      56 | static const unsigned int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
         |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix it by moving 'HPAGE_FRAG_CHECK_INTERVAL_MSEC' under
CONFIG_COMPACTION defconfig.

Also since this is just a 'static const int' type, use #define for it.

Link: https://lkml.kernel.org/r/1647608518-20924-1-git-send-email-quic_charante@quicinc.com
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Nitin Gupta <nigupta@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Charan Teja Kalla

mm/compaction.c

@@ -26,6 +26,11 @@
 #include "internal.h"
 
 #ifdef CONFIG_COMPACTION
+/*
+ * Fragmentation score check interval for proactive compaction purposes.
+ */
+#define HPAGE_FRAG_CHECK_INTERVAL_MSEC	(500)
+
 static inline void count_compact_event(enum vm_event_item item)
 {
 	count_vm_event(item);
@@ -50,11 +55,6 @@ static inline void count_compact_events(enum vm_event_item item, long delta)
 #define pageblock_start_pfn(pfn)	block_start_pfn(pfn, pageblock_order)
 #define pageblock_end_pfn(pfn)		block_end_pfn(pfn, pageblock_order)
 
-/*
- * Fragmentation score check interval for proactive compaction purposes.
- */
-static const unsigned int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
-
 /*
  * Page order with-respect-to which proactive compaction
  * calculates external fragmentation, which is used as