bpf: fix bpf_dynptr_slice() to stop return an ERR_PTR.

Verify if the pointer obtained from bpf_xdp_pointer() is either an error or
NULL before returning it.

The function bpf_dynptr_slice() mistakenly returned an ERR_PTR. Instead of
solely checking for NULL, it should also verify if the pointer returned by
bpf_xdp_pointer() is an error or NULL.
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/d1360219-85c3-4a03-9449-253ea905f9d1@moroto.mountain/
Fixes: 66e3a13e ("bpf: Add bpf_dynptr_slice and bpf_dynptr_slice_rdwr")
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/20230803231206.1060485-1-thinker.li@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/helpers.c

@@ -2270,7 +2270,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
 	case BPF_DYNPTR_TYPE_XDP:
 	{
 		void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);
-		if (xdp_ptr)
+		if (!IS_ERR_OR_NULL(xdp_ptr))
 			return xdp_ptr;
 
 		if (!buffer__opt)