x86/mce: Reuse one of the u16 padding fields in 'struct mce'

... to save the error severity of the MCE and whether the
reported address of the error is usable.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1439396985-12812-2-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>