bpf: bpf_stackmap_copy depends on CONFIG_PERF_EVENTS

0-day bot reported build error:
kernel/built-in.o: In function `map_lookup_elem':
>> kernel/bpf/.tmp_syscall.o:(.text+0x329b3c): undefined reference to `bpf_stackmap_copy'
when CONFIG_BPF_SYSCALL is set and CONFIG_PERF_EVENTS is not.
Add weak definition to resolve it.
This code path in map_lookup_elem() is never taken
when CONFIG_PERF_EVENTS is not set.

Fixes: 557c0c6e7d ("bpf: convert stackmap to pre-allocation")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/syscall.c

@@ -244,6 +244,11 @@ static void __user *u64_to_ptr(__u64 val)
 	return (void __user *) (unsigned long) val;
 }
 
+int __weak bpf_stackmap_copy(struct bpf_map *map, void *key, void *value)
+{
+	return -ENOTSUPP;
+}
+
 /* last field in 'union bpf_attr' used by this command */
 #define BPF_MAP_LOOKUP_ELEM_LAST_FIELD value