Handle lack of DSM slots in parallel btree build, take 2.
Commit
74618e77 added a new check intended to fix a bug, but put
it in the wrong place so that parallel btree build was always
disabled. Do the check after we've actually tried to create
a DSM segment. Back-patch to 11, like the earlier commit.
Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzmDABkJzrNnvf%2BOULK-_A_j9gkYg_Dz-H62jzNv4eKQTw%40mail.gmail.com