kni: add build option to disable preempting

kni: add build option to disable preempting

This patch introduces CONFIG_RTE_KNI_PREEMPT_DEFAULT flag. When set to 'no',
KNI kernel thread(s) do not call schedule_timeout_interruptible(), which
improves overall KNI performance at the expense of CPU cycles (polling).

Default values is 'yes', maintaining the same behaviour as of now.

Signed-off-by: Marc Sune <marc.sune@bisdn.de>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>