cxgbe(4): Add a driver ioctl to set the filter mask.

Allow the filter mask (aka the hashfilter mode when hashfilters are
in use) to be set any time it is safe to do so. The requested mask
must be a subset of the filter mode already. The driver will not change
the mode or ingress config just to support a new mask.

MFC after: 2 weeks
Sponsored by: Chelsio Communications