5.4 backport of io_uring/io-wq fix

Hi,

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!