lib/ovs-rcu: evaluate argument of ovsrcu_get only once.

lib/ovs-rcu: evaluate argument of ovsrcu_get only once.

As ovsrcu_get() looks like a function call, it is reasonable for the
callers to expect that the arguments are evaluated only once.
CONST_CAST expands its 'POINTER' argument multiple times, and the
exact effect of this turned out to be compiler dependent.  Fix this by
expanding the macro argument before CONST_CAST, and removing
unnecessary CONST_CASTs.

VMware-BZ: #1287651

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>