io_uring: stash ctx task reference instead of task files

io_uring: stash ctx task reference instead of task files

We can grab a reference to the task instead of stashing away the task
files_struct. This is doable without creating a circular reference
between the ring fd and the task itself.

This is in preparation for handling the ->files assignment a bit
differently, so we don't need to force SQPOLL to enter the kernel for
an update.

Signed-off-by: Jens Axboe <axboe@kernel.dk>