cleanup smgr.c of tablespace call

Hi,

This trivial patch moves a misplaced tablespace.c call from smgr.c into
the newly created storage.c.  This is appropriate because smgr.c is
supposed to be bare metal, and storage.c has the high-level calls.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support