cmap: Fix memory ordering for counter_changed().

Release memory ordering only affects visibility of stores, and is not
allowed on a memory read.  Some compilers enforce this, making this code
fail to compile.

Reported-by: Alex Wang <alexw@nicira.com>
Reported-by: Kmindg G <kmindg@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>