tg3: Use napi_alloc_frag()

tg3_alloc_rx_data() uses netdev_alloc_frag() for skb allocation. All
callers of tg3_alloc_rx_data() either hold tp->lock (which is held with
BH disabled) or run in NAPI context.

Use napi_alloc_frag() for skb allocations.

Cc: Siva Reddy Kallam <siva.kallam@broadcom.com>
Cc: Prashant Sreedharan <prashant@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>