fix the following warning
../kernel/bpf/syscall.c: In function ‘map_lookup_and_delete_elem’:
../kernel/bpf/syscall.c:1010:22: warning: unused variable ‘ptr’ [-Wunused-variable]
  void *key, *value, *ptr;
                      ^~~
Fixes: bd513cd08f10 ("bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
        void __user *uvalue = u64_to_user_ptr(attr->value);
        int ufd = attr->map_fd;
        struct bpf_map *map;
-       void *key, *value, *ptr;
+       void *key, *value;
        u32 value_size;
        struct fd f;
        int err;