gluster/uss: Handle notify in snapview-client

As there are two subvolumes in snapview-client, there is
a possibility that the regular subvolume is still down and
snapd subvolume come up first. So if we don't handle this situation
CHILD_UP event will be propagated upwards to fuse when regular subvolume
is still down. This can cause data unavailable for the application

Change-Id: I9e5166ed22c2cf637c15db0457c2b57ca044078e
BUG: 1168643
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/9205
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>