netpoll: Add netpoll_tx_running

This patch adds the helper netpoll_tx_running for use within
ndo_start_xmit.  It returns non-zero if ndo_start_xmit is being
invoked by netpoll, and zero otherwise.

This is currently implemented by simply looking at the hardirq
count.  This is because for all non-netpoll uses of ndo_start_xmit,
IRQs must be enabled while netpoll always disables IRQs before
calling ndo_start_xmit.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>