cluster/afr: Avoid priv->eager_lock value update race

Change-Id: I7049c0c64e36a9dfa4cc0e0b34de7ec111d2f6c1
BUG: 908302
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4076
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>