ixgbe: add IPv6 mask for flow director

Write FDIRIP6M register to allow flow director filter
to set ipv6 rules without setting ipv6 source/destination address.

Signed-off-by: Piotr Skajewski <piotrx.skajewski@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>

Approved by: imp
Obtained from: DPDK (21feefa2fcd5899ee26a10be405c17c0a1109860)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621

(cherry picked from commit c4f73d5d6a6c0117e08a03920cce69202865ba13)