kernel/fork.c: make max_threads symbol static

Fix build warning,
kernel/fork.c:125:5: warning: symbol 'max_threads' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20190516015118.140561-1-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -123,7 +123,7 @@
 unsigned long total_forks;	/* Handle normal Linux uptimes. */
 int nr_threads;			/* The idle threads do not count.. */
 
-int max_threads;		/* tunable limit on nr_threads */
+static int max_threads;		/* tunable limit on nr_threads */
 
 DEFINE_PER_CPU(unsigned long, process_counts) = 0;