This fixes three locations in net/ where per-CPU data could bite us
under preemption.  This is the result of an audit I did and should
constitute all of the unsafe code in net/.

In net/core/skbuff.c I did not have to introduce any code - just
rearrange the grabbing of smp_processor_id() to be in the interrupt off
region.  Pretty clean fixes.

Note in the future we can use put_cpu() and get_cpu() to grab the CPU#
safely.  I will send a patch to Marcelo so we can have a 2.4 version
(which doesn't do the preempt stuff), too...