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