rgw: set bucket attrs are a bucket instance meta operation

rgw: set bucket attrs are a bucket instance meta operation

Need to do the action through the bucket instance handler
and not through the bucket handler, otherwise it's wrongly
recorded (and wrongly replayed, ouch).

Fixes: #5791
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>