f2fs: pids_lock can be static

fs/f2fs/trace.c:19:12: sparse: symbol 'pids_lock' was not declared. Should it be static?
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/trace.c

@@ -17,7 +17,7 @@
 #include "trace.h"
 
 RADIX_TREE(pids, GFP_ATOMIC);
-spinlock_t pids_lock;
+static spinlock_t pids_lock;
 struct last_io_info last_io;
 
 static inline void __print_last_io(void)