pgsql: Make safeguard against incorrect flags for fsync more portable.

Make safeguard against incorrect flags for fsync more portable.

The existing code assumed that O_RDONLY is defined as 0, but this is
not required by POSIX and is not true on GNU Hurd. We can avoid
the assumption by relying on O_ACCMODE to mask the fcntl() result.
(Hopefully, all supported platforms define that.)

Author: Michael Banck <mbanck(at)gmx(dot)net>
Co-authored-by: Samuel Thibault
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/6862e8d1.050a0220.194b8d.76fa@mx.google.com
Discussion: https://postgr.es/m/68480868.5d0a0220.1e214d.68a6@mx.google.com
Backpatch-through: 13

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3a2617e4f0be734f58f7219986c323469b502921

Modified Files
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src/backend/storage/file/fd.c | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)