pgsql: Fix wal_writer_flush_after initializer value.

Fix wal_writer_flush_after initializer value.

Commit a73952b7956 (new in 16) required default values in guc_table.c
and C variable initializers to match.  This one only matched when
XLOG_BLCKSZ == 8kB.  Fix by using the same expression in both places
with a new DEFAULT_XXX macro, as done for other GUCs.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGLNmLV=VrT==5MqnbARgx2ifRSFtdd8ofdfrdSLL3yv5A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/63932a6d38e5dfa6df2a51a04b7314ec1e4d3de7

Modified Files
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src/backend/postmaster/walwriter.c  | 2 +-
src/backend/utils/misc/guc_tables.c | 2 +-
src/include/postmaster/walwriter.h  | 2 ++
3 files changed, 4 insertions(+), 2 deletions(-)