perf/bpf: fix a clang compilation issue

clang does not support variable length array for structure member.
It has the following error during compilation:

kernel/trace/trace_syscalls.c:568:17: error: fields must have a constant size:
'variable length array in structure' extension will never be supported
                unsigned long args[sys_data->nb_args];
                              ^

The fix is to use a fixed array length instead.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 88951b7..95606a2 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -200,6 +200,8 @@
 #define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
 #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
 
+#define SYSCALL_DEFINE_MAXARGS	6
+
 #define SYSCALL_DEFINEx(x, sname, ...)				\
 	SYSCALL_METADATA(sname, x, __VA_ARGS__)			\
 	__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 9c4eef2..696afe7 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -565,7 +565,7 @@
 	struct syscall_tp_t {
 		unsigned long long regs;
 		unsigned long syscall_nr;
-		unsigned long args[sys_data->nb_args];
+		unsigned long args[SYSCALL_DEFINE_MAXARGS];
 	} param;
 	int i;