rgw: fix multi delete

Fixes: #5931
Backport: bobtail, cuttlefish

Fix a bad check, where we compare the wrong field. Instead of
comparing the ret code to 0, we compare the string value to 0
which generates implicit casting, hence the crash.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>