xtensa: enforce slab alignment to maximum register width

XCHAL_DATA_WIDTH is the maximum register width, slab caches should be
aligned to this.

Theoretical fix as all variants have had an XCHAL_DATA_WIDTH of 4
(wordsize) for now.  But the S6000 variant will raise this to 16.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/include/asm/processor.h

@@ -25,6 +25,8 @@
 # error Linux requires the Xtensa Windowed Registers Option.
 #endif
 
+#define ARCH_SLAB_MINALIGN	XCHAL_DATA_WIDTH
+
 /*
  * User space process size: 1 GB.
  * Windowed call ABI requires caller and callee to be located within the same