rgw: return 423 Locked response when failing to lock object

Fixes: #5882
Translate the EBUSY we get when trying to lock a shard / object
to 423 Locked response. Beforehand it was just translated to the
default 500.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>