io_uring: move to separate directory

[ Upstream commit ed29b0b4

 ]

In preparation for splitting io_uring up a bit, move it into its own
top level directory. It didn't really belong in fs/ anyway, as it's
not a file system only API.

This adds io_uring/ and moves the core files in there, and updates the
MAINTAINERS file for the new location.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 998b30c3

 ("io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()")
Signed-off-by: Sasha Levin <sashal@kernel.org>