igb_uio: add write combining option
igb_uio: add write combining option

Write combining (WC) increases NIC performance by making better
utilization of PCI bus, but cannot be use by all PMD.

To get internal_addr memory need to be mapped. But as memory could not be
mapped twice: with and without WC it should be skipped for WC. [1]

To do not spoil other drivers that potentially could use internal_addr,
parameter wc_activate adds possibility to skip it for those PMDs,
that do not use it.

[1] https://www.kernel.org/doc/ols/2008/ols2008v2-pages-135-144.pdf
	section 5.3 and 5.4

Signed-off-by: Rafal Kozik <6265a4a07968a7c1df16a61004fb7191177cafd4@semihalf.com>
Acked-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Acked-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk