pgsql: Fix aboriginal bug in BufFileDumpBuffer that would cause it to

Log Message:
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Fix aboriginal bug in BufFileDumpBuffer that would cause it to write the
wrong data when dumping a bufferload that crosses a component-file boundary.
This probably has not been seen in the wild because (a) component files are
normally 1GB apiece and (b) non-block-aligned buffer usage is relatively
rare.  But it's fairly easy to reproduce a problem if one reduces RELSEG_SIZE
in a test build.  Kudos to Kurt Harriman for spotting the bug.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/storage/file:
        buffile.c (r1.24 -> r1.24.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/buffile.c.diff?r1=1.24&r2=1.24.2.1)