OSDMonitor: set next commit in mon primary-affinity reply

Commit 8c5c55c8b47e ("mon: set next commit in mon command replies")
fixed MMonCommand replies to include the right version, but the
primary-affinity handler was authored before that.  Fix it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>