Always grab work environment for deferred links. The assumption that we
will be running it always from the task in question is false, as exiting
tasks may mean that we're deferring this one to a thread helper. And at
that point it's too late to grab the work environment.

Fixes: debb85f4 ("io_uring: factor out grab_env() from defer_prep()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>