pgsql: Fix ReadBuffer() to correctly handle the case where it's trying

Log Message:
-----------
Fix ReadBuffer() to correctly handle the case where it's trying to extend
the relation but it finds a pre-existing valid buffer. The buffer does not
correspond to any page known to the kernel, so we *must* do smgrextend to
ensure that the space becomes allocated. The 7.x branches all do this
correctly, but the corner case got lost somewhere during 8.0 bufmgr rewrites.
(My fault no doubt :-( ... I think I assumed that such a buffer must be
not-BM_VALID, which is not so.)

Modified Files:
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pgsql/src/backend/storage/buffer:
bufmgr.c (r1.201 -> r1.202)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.201&r2=1.202)