pgsql: Add RelationOpenSmgr() calls to ensure rd_smgr is valid when we

Log Message:
-----------
Add RelationOpenSmgr() calls to ensure rd_smgr is valid when we try to
use it.  While it normally has been opened earlier during btree index
build, testing shows that it's possible for the link to be closed again
if an sinval reset occurs while the index is being built.

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtsort.c (r1.96 -> r1.97)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsort.c.diff?r1=1.96&r2=1.97)