rgw: only look at next placement rule if we're not at the last rule

Fixes: #7978
We tried to move to the next placement rule, but we were already at the
last one, so we ended up looping forever.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1599 from ceph/wip-7978

rgw: only look at next placement rule if we're not at the last rule

Reviewed-by: Sage Weil <sage@inktank.com>
Fix byte-order dependency in calculation of initial challenge

Fixes: #7977
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
doc/release-notes: v0.79 release notes

Signed-off-by: Sage Weil <sage@inktank.com>