openvswitch: use const in some local vars and casts

In few functions, const formal parameters are assigned or cast to
non-const.
These changes suppress warnings if compiled with -Wcast-qual.

Signed-off-by: Daniele Di Proietto <daniele.di.proietto@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>

diff --cc net/openvswitch/flow_netlink.c
Simple merge

diff --cc net/openvswitch/flow_table.c
Simple merge