tg3: Use slab_build_skb() when needed

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 ce098da1497c ("skbuff: Introduce
slab_build_skb()"):

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

Fixes: ce098da1497c ("skbuff: Introduce slab_build_skb()")
Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Closes: https://lore.kernel.org/all/1bd4cb9c-4eb8-3bdb-3e05-8689817242d1@proxmox.com
Cc: Siva Reddy Kallam <siva.kallam@broadcom.com>
Cc: Prashant Sreedharan <prashant@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230818175417.never.273-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>