io_uring: prevent reg-wait speculations

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

io_uring: prevent reg-wait speculations

With *ENTER_EXT_ARG_REG instead of passing a user pointer with arguments

for the waiting loop the user can specify an offset into a pre-mapped

region of memory, in which case the

[offset, offset + sizeof(io_uring_reg_wait)) will be intepreted as the

argument.

As we address a kernel array using a user given index, it'd be a subject

to speculation type of exploits. Use array_index_nospec() to prevent

that. Make sure to pass not the full region size but truncate by the

maximum offset allowed considering the structure size.