Changing hash used for selecting bucket in a group action

Current hash uses just the dl_dst field. This patch expands the hash  to
include all L2, L3 and L4 fields, allowing for more balanced selection.

Signed-off-by: Srini Seetharaman <srini.seetharaman@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>