mm/swap_slots.c: make swap_slots_cache_mutex and swap_slots_cache_enable_mutex static

The mutexes swap_slots_cache_mutex and swap_slots_cache_enable_mutex are
local to the source and do not need to be in global scope, so make them
static.

Cleans up sparse warnings:
  symbol 'swap_slots_cache_mutex' was not declared. Should it be static?
  symbol 'swap_slots_cache_enable_mutex' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20180624182536.4937-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_slots.c

@@ -38,9 +38,9 @@ static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
 static bool	swap_slot_cache_active;
 bool	swap_slot_cache_enabled;
 static bool	swap_slot_cache_initialized;
-DEFINE_MUTEX(swap_slots_cache_mutex);
+static DEFINE_MUTEX(swap_slots_cache_mutex);
 /* Serialize swap slots cache enable/disable operations */
-DEFINE_MUTEX(swap_slots_cache_enable_mutex);
+static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
 
 static void __drain_swap_slots_cache(unsigned int type);
 static void deactivate_swap_slots_cache(void);