io-wq: clear out worker ->fs and ->files

By default, kernel threads have init_fs and init_files assigned. In the
past, this has triggered security problems, as commands that don't ask
for (and hence don't get assigned) fs/files from the originating task
can then attempt path resolution etc with access to parts of the system
they should not be able to.

Rather than add checks in the fs code for misuse, just set these to
NULL. If we do attempt to use them, then the resulting code will oops
rather than provide access to something that it should not permit.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>