bpf: fix inconsistent return types of bpf_xdp_copy_buf().

Fix inconsistent return types in two implementations of bpf_xdp_copy_buf().

There are two implementations: one is an empty implementation whose return
type does not match the actual implementation.
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230804005101.1534505-1-thinker.li@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

include/linux/filter.h

@@ -1572,10 +1572,9 @@ static inline void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
 	return NULL;
 }
 
-static inline void *bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, void *buf,
-				     unsigned long len, bool flush)
+static inline void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, void *buf,
+				    unsigned long len, bool flush)
 {
-	return NULL;
 }
 #endif /* CONFIG_NET */