Increase the TSS limit by one byte. The processor requires an additional byte
with all bits set to 1 beyond the I/O permission bitmap.

Prior to this change accessing I/O ports [0xFFF8-0xFFFF] would trigger a
#GP fault even though the I/O bitmap allowed access to those ports.

For more details see section "I/O Permission Bit Map" in the Intel SDM, Vol 1.

Reviewed by:	kib