libceph: don't omit used_replica in target_copy()

Currently target_copy() is used only for sending linger pings, so
this doesn't come up, but generally omitting used_replica can hang
the client as we wouldn't notice the acting set change (legacy_change
in calc_target()) or trigger a warning in handle_reply().

Fixes: 117d96a0 ("libceph: support for balanced and localized reads")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>