io_uring: ensure '0' is returned on file registration success

A previous backport mistakenly removed code that cleared 'ret' to zero,
as the SCM logging was performed. Fix up the return value so we don't
return an errant error on fixed file registration.

Fixes: a6771f34 ("io_uring: drop any code related to SCM_RIGHTS")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>