tg3: use netdev_alloc_frag() API

Update our reference driver to use netdev_alloc_frag() API instead of
the temporary custom allocator I introduced in commit 8d4057a938
(tg3: provide frags as skb head)

This removes the memory leak we had, since we could leak one page at
device dismantle.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Matt Carlson <mcarlson@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>