net: remove NR_CPUS arrays in net/core/dev.c

Remove the fixed size channels[NR_CPUS] array in net/core/dev.c and
dynamically allocate array based on nr_cpu_ids.

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>