x86/tss: Fix and move VMX BUILD_BUG_ON()

The BUILD_BUG_ON(IO_BITMAP_OFFSET - 1 == 0x67) in the VMX code is bogus in
two aspects:

1) This wants to be in generic x86 code simply to catch issues even when
   VMX is disabled in Kconfig.

2) The IO_BITMAP_OFFSET is not the right thing to check because it makes
   asssumptions about the layout of tss_struct. Nothing requires that the
   I/O bitmap is placed right after x86_tss, which is the hardware mandated
   tss structure. It pointlessly makes restrictions on the struct
   tss_struct layout.

The proper thing to check is:

    - Offset of x86_tss in tss_struct is 0
    - Size of x86_tss == 0x68

Move it to the other build time TSS checks and make it do the right thing.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Andy Lutomirski <luto@kernel.org>