igb: add XDP support

Add XDP support to the IGB driver.
The implementation follows the IXGBE XDP implementation
closely and I used the following patches as basis:

1. commit 92470808 ("ixgbe: add XDP support for pass and drop actions")
2. commit 33fdc82f ("ixgbe: add support for XDP_TX action")
3. commit ed93a398

 ("ixgbe: tweak page counting for XDP_REDIRECT")

Due to the hardware constraints of the devices using the
IGB driver we must share the TX queues with XDP which
means locking the TX queue for XDP.

I ran tests on an older device to get better numbers.
Test machine:

Intel(R) Atom(TM) CPU C2338 @ 1.74GHz (2 Cores)
2x Intel I211

Routing Original Driver Network Stack: 382 Kpps

Routing XDP Redirect (xdp_fwd_kern): 1.48 Mpps
XDP Drop: 1.48 Mpps

Using XDP we can achieve line rate forwarding even on
an older Intel Atom CPU.

Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>