bpf: Return an error pointer for skb metadata when CONFIG_NET=n

Kernel Test Robot reported a compiler warning - a null pointer may be
passed to memmove in __bpf_dynptr_{read,write} when building without
networking support.

The warning is correct from a static analysis standpoint, but not actually
reachable. Without CONFIG_NET, creating dynptrs to skb metadata is
impossible since the constructor kfunc is missing.

Silence the false-postive diagnostic message by returning an error pointer
from bpf_skb_meta_pointer stub when CONFIG_NET=n.

Fixes: 6877cd3 ("bpf: Enable read/write access to skb metadata through a dynptr")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>

Author: jsitnicki

include/linux/filter.h

@@ -1816,7 +1816,7 @@ static inline void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, voi
 
 static inline void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 #endif /* CONFIG_NET */