io_uring: convert io_uring to the secure anon inode interface

Converting io_uring's anonymous inode to the secure anon inode API
enables LSMs to enforce policy on the io_uring anonymous inodes if
they chose to do so.  This is an important first step towards
providing the necessary mechanisms so that LSMs can apply security
policy to io_uring operations.

Signed-off-by: Paul Moore <paul@paul-moore.com>