io_uring/msg_ring: avoid double indirection task_work for fd passing

io_uring/msg_ring: avoid double indirection task_work for fd passing

Like what was done for MSG_RING data passing avoiding a double task_work
roundtrip for IORING_SETUP_DEFER_TASKRUN, implement the same model for
fd passing. File descriptor passing is separately locked anyway, so the
only remaining issue is CQE posting, just like it was for data passing.
And for that, we can use the same approach.

Signed-off-by: Jens Axboe <axboe@kernel.dk>