pgsql: Adjust pg_fsync_writethrough so that it will set errno when

Log Message:
-----------
Adjust pg_fsync_writethrough so that it will set errno when failing
on a platform that doesn't support this operation.  The former coding
would allow an unrelated errno to be reported, which would be quite
misleading.  Not sure if this has anything to do with the current
buildfarm failures, but it's certainly bogus as-is.

Modified Files:
--------------
    pgsql/src/backend/storage/file:
        fd.c (r1.154 -> r1.155)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c?r1=1.154&r2=1.155)