x86/ioperm: Move iobitmap data into a struct

No point in having all the data in thread_struct, especially as upcoming
changes add more.

Make the bitmap in the new struct accessible as array of longs and as array
of characters via a union, so both the bitmap functions and the update
logic can avoid type casts.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>