io_uring: grab ->fs as part of async offload

[ Upstream commits 9392a27d88b9 and ff002b30181d ]

Ensure that the async work grabs ->fs from the queueing task if the
punted commands needs to do lookups.

We don't have these two commits in 5.4-stable:

ff002b30181d30cdfbca316dadd099c3ca0d739c
9392a27d88b9707145d713654eb26f0c29789e50

because they don't apply with the rework that was done in how io_uring
handles offload. Since there's no io-wq in 5.4, it doesn't make sense to
do two patches. I'm attaching my port of the two for 5.4-stable, it's
been tested. Please queue it up for the next 5.4-stable, thanks!

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>