io_uring: ensure consistent view of original task ->mm from SQPOLL

Ensure we get a valid view of the task mm, by using task_lock() when
attempting to grab the original task mm.

Reported-by:  <syzbot+b57abf7ee60829090495@syzkaller.appspotmail.com>
Fixes: 2aede0e4 ("io_uring: stash ctx task reference for SQPOLL")
Signed-off-by: Jens Axboe <axboe@kernel.dk>