Incorrect version number given to sync_pgdata() in pg_combinebackup.c

Hi all,

While hacking on a different thing that touched pg_combinebackup, I
have bumped into a silly bug.

To keep it short, the version number is calculated based on this code
in read_pg_version_file(), where "version" is the result of strtoul()
applied to the contents of PG_VERSION:
    return version * 10000;

For v18, this would return 180000, which is fine.

A bit later on, we do that, which is not fine:
    sync_pgdata(opt.output, version * 10000, opt.sync_method, true);

This leads to a version number higher than expected, multiplied twice.
This was harmless, because sync_pgdata uses the version number to make
the difference between pg_wal/ and pg_xlog/, and pg_combinebackup does
not support versions older than v10, which is exactly where the
renamed happened.  Hence, even if the version number was too high, we
always expect to flush pg_wal/.

Trivial patch attached, for a backpatch down to where pg_combinebackup
has been introduced.

Thoughts?
--
Michael