ubd: Use pointer-to-pointers for io_thread_req arrays

ubd: Use pointer-to-pointers for io_thread_req arrays

Having an unbounded array for irq_req_buffer and io_req_buffer doesn't
provide any bounds safety, and confuses the needed allocation type,
which is returning a pointer to pointers. Instead of the implicit cast,
switch the variable types.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/all/b04b6c13-7d0e-4a89-9e68-b572b6c686ac@roeck-us.net
Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
Acked-by: Richard Weinberger <richard@nod.at>
Link: https://patch.msgid.link/20260223214341.work.846-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>