seccomp: Replace BUG(!spin_is_locked()) with assert_spin_lock

Current upstream kernel hangs with mips and powerpc targets in
uniprocessor mode if SECCOMP is configured.

Bisect points to commit dbd952127d ("seccomp: introduce writer locking").
Turns out that code such as
	BUG_ON(!spin_is_locked(&list_lock));
can not be used in uniprocessor mode because spin_is_locked() always
returns false in this configuration, and that assert_spin_locked()
exists for that very purpose and must be used instead.

Fixes: dbd952127d ("seccomp: introduce writer locking")
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/fork.c

@@ -1105,7 +1105,7 @@ static void copy_seccomp(struct task_struct *p)
 	 * needed because this new task is not yet running and cannot
 	 * be racing exec.
 	 */
-	BUG_ON(!spin_is_locked(&current->sighand->siglock));
+	assert_spin_locked(&current->sighand->siglock);
 
 	/* Ref-count the new filter user, and assign it. */
 	get_seccomp_filter(current);