Document `ifnetlist' locking.

Document `ifnetlist' locking.

We use both kernel and net lock for protect `ifnetlist'. This means we
do modification with both locks held, but for read-only access only one
lock required. Some places doing `ifnetlist' foreach loop are protected
by kernel lock and context switch can't be introduced there. This is the
exception, so "XXXSMP:" comment added.

Proposed and ok by bluhm@