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.)

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.185 -> r1.185.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.185&r2=1.185.4.1)