rt: fix io_uring shutdown errors

[ThanhDodeurOdoo]

Shutdown errors can leave operations pending. Panicking then drops
operation resources that the kernel may still be using.

Drain available completions before retrying EINTR, EBUSY or EAGAIN
from shutdown waits. Also retry EINTR during submission .

Before panicking on other shutdown errors, leak thé operation slab
to keep pending buffers alive (same for submission
flush failures).

[ThanhDodeurOdoo]

will write a test, just waiting for feedback before

[Darksonn]

Thanks for catching this! Left two comments on the error handling in UringContext::drop — once those are addressed, please also go ahead and add a test in tokio/tests/fs_uring.rs.

[ThanhDodeurOdoo]

just added a test ( a FIFO to keep I/O pending). I checked that fails without the fix