io_uring: flag the fact that linked file assignment is sane

Give applications a way to tell if the kernel supports sane linked files,
as in files being assigned at the right time to be able to reliably
do <open file direct into slot X><read file from slot X> while using
IOSQE_IO_LINK to order them.

Not really a bug fix, but flag it as such so that it gets pulled in with
backports of the deferred file assignment.

Fixes: 6bf9c47a ("io_uring: defer file assignment")
Signed-off-by: Jens Axboe <axboe@kernel.dk>