pgsql-server: FlushRelationBuffers was also being a bit cavalier about

Log Message:
-----------
FlushRelationBuffers was also being a bit cavalier about whether the
relation is already opened by smgr.

Modified Files:
--------------
pgsql-server/src/backend/storage/buffer:
bufmgr.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/buffer/bufmgr.c.diff?r1=1.175&r2=1.176)