datapath: Add loop detection for RT kernels.

datapath: Add loop detection for RT kernels.

Our normal loop detection requires disabling preemption while
packet processing takes place.  On RT kernels this isn't acceptable
and interacts badly with spinlocks, so we can't use it.  This
takes advantage of some extra space that is added to struct
task_struct on RT kernels (and the knowledge that we will always
have a valid task_struct) to store the loop counter for a given
thread.  Since we can't make these assumptions on non-RT kernels,
we continue to use the previous method of loop detection there.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>