x86/fpu/mpx: Rework MPX 'xstate' types

x86/fpu/mpx: Rework MPX 'xstate' types

MPX includes two separate "extended state components".  There is
no real need to have an 'mpx_struct' because we never really
manage the states together.

We also separate out the actual data in 'mpx_bndcsr_state' from
the padding.  We will shortly be checking the state sizes
against our structures and need them to match.  For consistency,
we also ensure to prefix these types with 'mpx_'.

Lastly, we add some comments to mirror some of the descriptions
in the Intel documents (SDM) of the various state components.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: dave@sr71.net
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20150902233129.384B73EB@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>