skbuff: Add likely to skb pointer in build_skb()

Similarly to napi_build_skb(), it is likely the skb allocation in
build_skb() succeeded. frag_size != 0 is also likely, as stated in
__build_skb_around().
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -420,7 +420,7 @@ struct sk_buff *build_skb(void *data, unsigned int frag_size)
 {
 	struct sk_buff *skb = __build_skb(data, frag_size);
 
-	if (skb && frag_size) {
+	if (likely(skb && frag_size)) {
 		skb->head_frag = 1;
 		skb_propagate_pfmemalloc(virt_to_head_page(data), skb);
 	}