pgsql: Rearrange mdsync() looping logic to avoid the problem that a

Log Message:
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Rearrange mdsync() looping logic to avoid the problem that a sufficiently
fast flow of new fsync requests can prevent mdsync() from ever completing.
This was an unforeseen consequence of a patch added in Mar 2006 to prevent
the fsync request queue from overflowing.  Problem identified by Heikki
Linnakangas and independently by ITAGAKI Takahiro; fix based on ideas from
Takahiro-san, Heikki, and Tom.

Back-patch as far as 8.1 because a previous back-patch introduced the problem
into 8.1 ...

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/storage/smgr:
        md.c (r1.123.2.1 -> r1.123.2.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/md.c.diff?r1=1.123.2.1&r2=1.123.2.2)