tg3: Use slab_build_skb() when needed

The tg3 driver will use kmalloc() under some conditions. Check the
frag_size and use slab_build_skb() when frag_size is 0. Silences
the warning introduced by commit ce098da ("skbuff: Introduce
slab_build_skb()"):

	Use slab_build_skb() instead
	...
	tg3_poll_work+0x638/0xf90 [tg3]

Fixes: ce098da ("skbuff: Introduce slab_build_skb()")
Reported-by: Fiona Ebner <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Cc: Siva Reddy Kallam <[email protected]>
Cc: Prashant Sreedharan <[email protected]>
Cc: Michael Chan <[email protected]>
Cc: Bagas Sanjaya <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Pavan Chebbi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/ethernet/broadcom/tg3.c

@@ -6881,7 +6881,10 @@ static int tg3_rx(struct tg3_napi *tnapi, int budget)
 
 			ri->data = NULL;
 
-			skb = build_skb(data, frag_size);
+			if (frag_size)
+				skb = build_skb(data, frag_size);
+			else
+				skb = slab_build_skb(data);
 			if (!skb) {
 				tg3_frag_free(frag_size != 0, data);
 				goto drop_it_no_recycle;