[NET] back: Make use of the simplicity of tasklets in net_rx_action

[NET] back: Make use of the simplicity of tasklets in net_rx_action

Tasklets have the property that each one is running on only one CPU at
any time.  This means that you don't have to worry about the tasklet
racing against itself.  Therefore any resources used by just a single
tasklet does not need to be guarded by locks.

Since net_rx_action is the only user of alloc_mfn, we can remove the
mfn_lock that guard it.

The notify_list array is huge by Linux standards so placing it on the
stack is unsafe.  Since net_rx_action is not re-entrant, we can simply
make it static.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>