rgw: quiet down ECANCELED on put_obj_meta()

Fixes: #5439

ECANCELED there means that we lost in a race to write the object. We
should treat it as a successful write. This is reviving an old behavior
that was changed inadvertently.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #439 from yehudasa/wip-rgw-next

rgw: quiet down ECANCELED on put_obj_meta()
Reviewed-by: Greg Farnum <greg@inktank.com>