It turns out that TablespaceCreateDbspace fails badly if a relcache flush

It turns out that TablespaceCreateDbspace fails badly if a relcache flush
occurs when it tries to heap_open pg_tablespace.  When control returns to
smgrcreate, that routine will be holding a dangling pointer to a closed
SMgrRelation, resulting in mayhem.  This is of course a consequence of
the violation of proper module layering inherent in having smgr.c call
a tablespace command routine, but the simplest fix seems to be to change
the locking mechanism.  There's no real need for TablespaceCreateDbspace
to touch pg_tablespace at all --- it's only opening it as a way of locking
against a parallel DROP TABLESPACE command.  A much better answer is to
create a special-purpose LWLock to interlock these two operations.
This drops TablespaceCreateDbspace quite a few layers down the food chain
and makes it something reasonably safe for smgr to call.