io_uring: refactor io_uring_allowed()

Have io_uring_allowed() return an error code directly instead of
true/false. This is needed for follow-up work to guard io_uring_setup()
with LSM.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
[PM: goto-to-return conversion as discussed on-list]
Signed-off-by: Paul Moore <paul@paul-moore.com>