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 
Reviewed-by: Michael Paquier 
Discussion: https://postgr.es/m/CA%2BhUKGLtsTUOScnNoSMZ-2ZLv%2BwGh01J6kAo_DM8mTRq1sKdSQ%40mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/052014a2066827cb96dbc9ef464ce44293585601

Modified Files
--------------
src/backend/access/transam/clog.c      | 7 -------
src/backend/access/transam/commit_ts.c | 6 ------
src/backend/access/transam/slru.c      | 4 ++++
3 files changed, 4 insertions(+), 13 deletions(-)