rgw/notification: fix zero eventTime in bucket notifications on concurrent PUT race

When concurrent PUTs target the same object key, RADOS may return
-ECANCELED to the losing writers. In that path *meta.mtime was never
populated from meta.set_mtime, leaving mtime at epoch (zero), which
propagated into bucket notification eventTime as
"1970-01-01T00:00:00.000Z".

Fix: set *meta.mtime from meta.set_mtime before returning 0 in the
ECANCELED/ENOENT/EEXIST early-return block, matching the behaviour of
the successful write path.

Also add a regression test that fires 20 concurrent threads writing
the same key and asserts no event in the persistent queue carries a
zero eventTime.

Fixes: https://tracker.ceph.com/issues/76729

Signed-off-by: Krunal Chheda <kchheda3@bloomberg.net>