Fix version number calculation for data folder flush in pg_combinebackup

Fix version number calculation for data folder flush in pg_combinebackup

The version number calculated by read_pg_version_file() is multiplied
once by 10000, to be able to do comparisons based on PG_VERSION_NUM or
equivalents with a minor version included.  However, the version number
given sync_pgdata() was multiplied by 10000 a second time, leading to an
overestimated number.

This issue was harmless (still incorrect) as pg_combinebackup does not
support versions of Postgres older than v10, and sync_pgdata() only
includes a version check due to the rename of pg_xlog/ to pg_wal/.  This
folder rename happened in the development cycle of v10.  This would
become a problem if in the future  sync_pgdata() is changed to have more
version-specific checks.

Oversight in dc212340058b, so backpatch down to v17.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aOil5d0y87ZM_wsZ@paquier.xyz
Backpatch-through: 17