io_uring/msg_ring: let target know allocated index

msg_ring requests transferring files support auto index selection via
IORING_FILE_INDEX_ALLOC, however they don't return the selected index
to the target ring and there is no other good way for the userspace to
know where is the receieved file.

Return the index for allocated slots and 0 otherwise, which is
consistent with other fixed file installing requests.

Cc: stable@vger.kernel.org # v6.0+
Fixes: e6130eba

 ("io_uring: add support for passing fixed file descriptors")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://github.com/axboe/liburing/issues/809
Signed-off-by: Jens Axboe <axboe@kernel.dk>