pgsql: Don't forget about failed fsync() requests.

Don't forget about failed fsync() requests.

If fsync() fails, md.c must keep the request in its bitmap, so that
future attempts will try again.

Back-patch to all supported releases.

Author: Thomas Munro
Reviewed-by: Amit Kapila
Reported-by: Andrew Gierth
Discussion: https://postgr.es/m/87y3i1ia4w.fsf%40news-spur.riddles.org.uk

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2b2010d12a62c4b77fdffbd988f5cbbcdcddb98e

Modified Files
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src/backend/storage/smgr/md.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)