datapath: #define local_bh_disable to preempt_disable on RT.
We use local_bh_disable() to mean that we don't want to get
preempted or interrupted and normally it covers both situations.
However, on RT kernels local_bh_disable() becomes a no-op because
bottom halves aren't interrupts any more. We still want the
behavior of disabling preemption so #define them to be the same
on these kernels.
Signed-off-by: Jesse Gross <jesse@nicira.com> Acked-by: Ben Pfaff <blp@nicira.com>