x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()

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

x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()

Commit 45e1dccc0653 ("x86/insn: Simplify for_each_insn_prefix()") made
for_each_insn_prefix() declare its own loop counter named idx.
get_seg_reg_override_idx() keeps its result in a local of the same name,
so the assignments in the loop body now write the loop counter:

- the function always returns INAT_SEG_REG_DEFAULT, so FS and GS
overrides are ignored;

- a CS override sets the counter to INAT_SEG_REG_CS (2), the increment
makes it 3, and prefixes.bytes[3] holds the last prefix, 0x2e again,
so the loop never terminates.

The UMIP emulation reaches this from the #GP handler for user space: an
SGDT, SIDT or SMSW with a memory operand and a CS prefix leaves the task
spinning in the kernel, never returning to user space, with a SIGKILL
that is never delivered. Any user can thus tie up CPUs until reboot.
With an FS or GS prefix, the emulated result is written to the wrong
address.

Rename the internal macro iterator variable.

[ Lukas' original patch renamed the use site, I changed it to rename the
iterator in the macro instead, but left Lukas' commit message and
authorship - Linus ]