We won't normally have a ton of flow masks but using a size_t to store
values no bigger than sizeof(struct sw_flow_key) seems excessive.

This reduces sw_flow_key_range and sw_flow_mask by 4 bytes on 32-bit
systems.  On 64-bit systems it shrinks sw_flow_key_range by 12 bytes but
sw_flow_mask only by 8 bytes due to padding.

Compile tested only.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>