bpf: Change kvfree to kfree in generic_map_lookup_batch()

buf_prevkey in generic_map_lookup_batch() is allocated with
kmalloc(). It's safe to free it with kfree().

Fixes: cb4d03ab49 ("bpf: Add generic support for lookup batch op")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200601162814.17426-1-efremov@linux.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/syscall.c

@@ -1399,7 +1399,7 @@ int generic_map_lookup_batch(struct bpf_map *map,
 
 	buf = kmalloc(map->key_size + value_size, GFP_USER | __GFP_NOWARN);
 	if (!buf) {
-		kvfree(buf_prevkey);
+		kfree(buf_prevkey);
 		return -ENOMEM;
 	}