Move all driver spinlocks to be taken at sw IRQ
context only.

This fixes the skb_copy() we were doing with hw
IRQs disabled (which is illegal and triggers a
BUG() with HIGHMEM enabled).  It also simplifies
the locking all over the driver tremendously.

We accomplish this feat by creating a special
sequence to synchronize with the hw IRQ handler
using a binary state and synchronize_irq().
This idea is from Herbert Xu.

Thanks to Michael Chan for helping to track down
all of the race conditions in initial versions
of this code.
Signed-off-by: David S. Miller <davem@davemloft.net>