kni: restrict bifurcated device support

kni: restrict bifurcated device support

To enable bifurcated device support, rtnl_lock is released before calling
userspace callbacks and asynchronous requests are enabled.

But these changes caused more issues, like bug #809, #816. To reduce the
scope of the problems, the bifurcated device support related changes are
only enabled when it is requested explicitly with new 'enable_bifurcated'
module parameter.
And bifurcated device support is disabled by default.

So the bifurcated device related problems are isolated and they can be
fixed without impacting all use cases.

Bugzilla ID: 816
Fixes: 631217c76135 ("kni: fix kernel deadlock with bifurcated device")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Igor Ryzhov <iryzhov@nfware.com>