io_uring: grab ->fs as part of async offload

io_uring: grab ->fs as part of async offload

[ Upstream commits 684451cacd8b and 3b74cfc349d5 ]

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:

3b74cfc349d5deff80a63f84db533022cbc58314
684451cacd8b117dc827b8b09fee5ef7323f3bf4

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>