cluster/distribute: get_layout should account only available subvols

The earlier logic used to check if (layout-spread-count <= subvol_cnt -
decommissioned bricks). With this if a subvol was down, and layout-spread was >
upsubvols, a mkdir ended up creating holes in the layout.

The fix is to consider only the combination of subvols which are usable (not
down or not decommissioned).

Change-Id: I61ad3bcaf4589f5a75f7887cfa595c98311ae3bb
BUG: 902610
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4412
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>