ixl(4): Update to use iflib

ixl(4): Update to use iflib

Update the driver to use iflib in order to bring performance,
maintainability, and (hopefully) stability benefits to the driver.

The driver currently isn't completely ported; features that are missing:

- VF driver (ixlv)
- SR-IOV host support
- RDMA support

The plan is to have these re-added to the driver before the next FreeBSD release.

Reviewed by:	gallatin@
Contributions by: gallatin@, mmacy@, krzysztof.galazka@intel.com
Tested by:	jeffrey.e.pieper@intel.com
MFC after:	1 month
Sponsored by:	Intel Corporation
Differential Revision:	https://reviews.freebsd.org/D15577