Thread: pgsql: Add a check to prevent overwriting valid data if smgrnblocks()

Log Message:
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Add a check to prevent overwriting valid data if smgrnblocks() gives a
wrong answer, as has been seen to occur with a buggy Linux kernel.  Not
really our bug, but it's a simple test in a seldom-used control path,
so might as well have a defense.

Modified Files:
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    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.210 -> r1.211)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.210&r2=1.211)