iflib: Allow drivers to determine which queue to TX on

Adds a new function pointer to struct if_txrx in order to allow
drivers to set their own function that will determine which queue
a packet should be sent on.

Since this includes a kernel ABI change, bump the __FreeBSD_version
as well.

(This motivation behind this is to allow the driver to examine the
UP in the VLAN tag and determine which queue to TX on based on
that, in support of HW TX traffic shaping.)

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: kbowling@, stallamr@netapp.com
Tested by: jeffrey.e.pieper@intel.com
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D31485