bnx2x: Use napi_alloc_frag()

SKB allocation via bnx2x_frag_alloc() is always performed in NAPI
context. Preemptible context passes GFP_KERNEL and bnx2x_frag_alloc()
uses then __get_free_page() for the allocation.

Use napi_alloc_frag() for memory allocation.

Cc: Ariel Elior <aelior@marvell.com>
Cc: Sudarsana Kalluru <skalluru@marvell.com>
Cc: GR-everest-linux-l2@marvell.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>