pgsql: Flush logical mapping files with fd opened for read/write at che

Flush logical mapping files with fd opened for read/write at checkpoint

The file descriptor was opened with read-only to fsync a regular file,
which would cause EBADFD errors on some platforms.

This is similar to the recent fix done by a586cc4b (which was broken by
me with 82a5649), except that I noticed this issue while monitoring the
backend code for similar mistakes. Backpatch to 9.4, as this has been
introduced since logical decoding exists as of b89e151.

Author: Michael Paquier
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20191006045548.GA14532@paquier.xyz
Backpatch-through: 9.4

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b8e19b932a99a7eb5a3bce84e74b0b7c093d0981

Modified Files
--------------
src/backend/access/heap/rewriteheap.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)