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>

fs/proc/array.c

@@ -341,6 +341,8 @@ static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
 	seq_put_decimal_ull(m, "NoNewPrivs:\t", task_no_new_privs(p));
 #ifdef CONFIG_SECCOMP
 	seq_put_decimal_ull(m, "\nSeccomp:\t", p->seccomp.mode);
+	seq_put_decimal_ull(m, "\nSeccomp_filters:\t",
+			    atomic_read(&p->seccomp.filter_count));
 #endif
 	seq_puts(m, "\nSpeculation_Store_Bypass:\t");
 	switch (arch_prctl_spec_ctrl_get(p, PR_SPEC_STORE_BYPASS)) {