fsstress: fix bogus compile warning do_renameat2()

fsstress: fix bogus compile warning do_renameat2()

gcc complains with:

fsstress.c:4629:4: warning: 'oldparid' may be used uninitialized in this function [-Wmaybe-uninitialized]
 4629 |    printf("%d/%d: rename source entry: id=%d,parent=%d\n",
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 4630 |     procid, opno, oldid, oldparid);
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fsstress.c:4629:4: warning: 'oldid' may be used uninitialized in this function [-Wmaybe-uninitialized]

But the varaibles are sure to be initialized, it is just that the
heuristics are broken since another check is used later which confuses
gcc. So just initialize the variables, to shup the compile warning.

Cc: kaixuxia <kaixuxia@tencent.com>
Cc: Brian Foster <bfoster@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>