io_probe checks io_issue_def->not_supported, but we never really set
that field, as we mark non-supported functions through a specific ->prep
handler.  This means we end up returning IO_URING_OP_SUPPORTED, even for
disabled operations.  Fix it by just checking the prep handler itself.

Fixes: 66f4af93 ("io_uring: add support for probing opcodes")
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20240619020620.5301-2-krisman@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>