dpif-netdev: Skip also xregs when building a mask.

dpif-netdev: Skip also xregs when building a mask.

When no mask is passed in, dpif_netdev_mask_from_nlattrs() builds a
mask from all possible fields whose prerequisites are met.  OpenFlow
metadata is not relevant for a datapath flow, so we skip registers and
metadata.  Do this for XREGs as well.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>