io_uring/tctx: work around xa_store() allocation error issue

In the Linux kernel, the following vulnerability has been resolved:

io_uring/tctx: work around xa_store() allocation error issue

syzbot triggered the following WARN_ON:

WARNING: CPU: 0 PID: 16 at io_uring/tctx.c:51 __io_uring_free+0xfa/0x140 io_uring/tctx.c:51

which is the

WARN_ON_ONCE(!xa_empty(&tctx->xa));

sanity check in __io_uring_free() when a io_uring_task is going through

its final put. The syzbot test case includes injecting memory allocation

failures, and it very much looks like xa_store() can fail one of its

memory allocations and end up with ->head being non-NULL even though no

entries exist in the xarray.

Until this issue gets sorted out, work around it by attempting to

iterate entries in our xarray, and WARN_ON_ONCE() if one is found.