x86/mpx: Introduce new 'directory entry' to 'addr' helper function

Currently, to get from a bounds directory entry to the virtual
address of a bounds table, we simply mask off a few low bits.
However, the set of bits we mask off is different for 32-bit and
64-bit binaries.

This breaks the operation out in to a helper function and also
adds a temporary variable to store the result until we are
sure we are returning one.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150607183704.007686CE@viggo.jf.intel.com


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