loci: only mask partially wildcarded fields

Reviewer: @ducwindow

The of_match_values_mask function was consuming about 7% CPU time in my 
benchmarks. For realistic flows most of the work it did was pointless, because 
most fields are fully wildcarded and most of the rest are not wildcarded. Only 
in the uncommon case of partially wildcarded fields does this function do 
anything.

Fixed by calling a new of_memmask function only when parsing a masked OXM.