pgsql: Fix missing fsync of SLRU directories.

Fix missing fsync of SLRU directories.

Harmonize behavior by moving reponsibility for fsyncing directories down
into slru.c.  In 10 and later, only the multixact directories were
missed (see commit 1b02be21), and in older branches all SLRUs were
missed.

Back-patch to all supported releases.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGLtsTUOScnNoSMZ-2ZLv%2BwGh01J6kAo_DM8mTRq1sKdSQ%40mail.gmail.com

Branch
------
REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/23d8b3577d0c247731addc188fe0479aa8f81c45

Modified Files
--------------
src/backend/access/transam/slru.c | 4 ++++
1 file changed, 4 insertions(+)