seccomp: Report number of loaded filters in /proc/$pid/status

A common question asked when debugging seccomp filters is "how many
filters are attached to your process?" Provide a way to easily answer
this question through /proc/$pid/status with a "Seccomp_filters" line.

Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/seccomp.h

@@ -13,6 +13,7 @@
 #ifdef CONFIG_SECCOMP
 
 #include <linux/thread_info.h>
+#include <linux/atomic.h>
 #include <asm/seccomp.h>
 
 struct seccomp_filter;
@@ -29,6 +30,7 @@ struct seccomp_filter;
  */
 struct seccomp {
 	int mode;
+	atomic_t filter_count;
 	struct seccomp_filter *filter;
 };