__netdev_alloc_skb() can be used from any context and is used by NAPI
and non-NAPI drivers. Non-NAPI drivers use it in interrupt context and
NAPI drivers use it during initial allocation (->ndo_open() or
->ndo_change_mtu()). Some NAPI drivers share the same function for the
initial allocation and the allocation in their NAPI callback.

The interrupts are disabled in order to ensure locked access from every
context to `netdev_alloc_cache'.

Let __netdev_alloc_skb() check if interrupts are disabled. If they are, use
`netdev_alloc_cache'. Otherwise disable BH and use `napi_alloc_cache.page'.
The IRQ check is cheaper compared to disabling & enabling interrupts and
memory allocation with disabled interrupts does not work on -RT.
Signed-off-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>