tracing/syscalls: Ignore numbers outside NR_syscalls' range

ARM has some private syscalls (for example, set_tls(2)) which lie
outside the range of NR_syscalls.  If any of these are called while
syscall tracing is being performed, out-of-bounds array access will
occur in the ftrace and perf sys_{enter,exit} handlers.

 # trace-cmd record -e raw_syscalls:* true && trace-cmd report
 ...
 true-653   [000]   384.675777: sys_enter:            NR 192 (0, 1000, 3, 4000022, ffffffff, 0)
 true-653   [000]   384.675812: sys_exit:             NR 192 = 1995915264
 true-653   [000]   384.675971: sys_enter:            NR 983045 (76f74480, 76f74000, 76f74b28, 76f74480, 76f76f74, 1)
 true-653   [000]   384.675988: sys_exit:             NR 983045 = 0
 ...

 # trace-cmd record -e syscalls:* true
 [   17.289329] Unable to handle kernel paging request at virtual address aaaaaace
 [   17.289590] pgd = 9e71c000
 [   17.289696] [aaaaaace] *pgd=00000000
 [   17.289985] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
 [   17.290169] Modules linked in:
 [   17.290391] CPU: 0 PID: 704 Comm: true Not tainted 3.18.0-rc2+ #21
 [   17.290585] task: 9f4dab00 ti: 9e710000 task.ti: 9e710000
 [   17.290747] PC is at ftrace_syscall_enter+0x48/0x1f8
 [   17.290866] LR is at syscall_trace_enter+0x124/0x184

Fix this by ignoring out-of-NR_syscalls-bounds syscall numbers.

Commit cd0980fc8add "tracing: Check invalid syscall nr while tracing syscalls"
added the check for less than zero, but it should have also checked
for greater than NR_syscalls.

Link: http://lkml.kernel.org/p/1414620418-29472-1-git-send-email-rabin@rab.in

Fixes: cd0980fc8add "tracing: Check invalid syscall nr while tracing syscalls"
Cc: stable@vger.kernel.org # 2.6.33+
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
static int perf_sysexit_enable(struct ftrace_event_call *call)
{
	int ret = 0;
	int num;

	num = ((struct syscall_metadata *)call->data)->syscall_nr;

	mutex_lock(&syscall_trace_lock);
	if (!sys_perf_refcount_exit)
		ret = register_trace_sys_exit(perf_syscall_exit, NULL);
	if (ret) {
		pr_info("event trace: Could not activate"
				"syscall exit trace point");
	} else {
		set_bit(num, enabled_perf_exit_syscalls);
		sys_perf_refcount_exit++;
	}
	mutex_unlock(&syscall_trace_lock);
	return ret;
}

static int perf_sysexit_enable(struct ftrace_event_call *call)
{
	int ret = 0;
	int num;

	num = ((struct syscall_metadata *)call->data)->syscall_nr;

	mutex_lock(&syscall_trace_lock);
	if (!sys_perf_refcount_exit)
		ret = register_trace_sys_exit(perf_syscall_exit, NULL);
	if (ret) {
		pr_info("event trace: Could not activate"
				"syscall exit trace point");
	} else {
		set_bit(num, enabled_perf_exit_syscalls);
		sys_perf_refcount_exit++;
	}
	mutex_unlock(&syscall_trace_lock);
	return ret;
}
