bnx2x: Fix busy_poll vs netpoll

Commit 9a2620c8 ("bnx2x: prevent WARN during driver unload")
switched the napi/busy_lock locking mechanism from spin_lock() into
spin_lock_bh(), breaking inter-operability with netconsole, as netpoll
disables interrupts prior to calling our napi mechanism.

This switches the driver into using atomic assignments instead of the
spinlock mechanisms previously employed.

Based on initial patch from Yuval Mintz & Ariel Elior

I basically added softirq starvation avoidance, and mixture
of atomic operations, plain writes and barriers.

Note this slightly reduces the overhead for this driver when no
busy_poll sockets are in use.

Fixes: 9a2620c8 ("bnx2x: prevent WARN during driver unload")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>