microblaze: Define ARCH_SLAB_MINALIGN to fix slab crash

The commit "mm: Move ARCH_SLAB_MINALIGN and
ARCH_KMALLOC_MINALIGN to <linux/slab_def.h>"
1f0ce8b3 which moved the ARCH_SLAB_MINALIGN
default into the global header broke FLAT for Microblaze.

Error message:
slab error in verify_redzone_free(): cache `idr_layer_cache':
memory outside object was overwritten
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/page.h

@@ -34,6 +34,8 @@
 /* MS be sure that SLAB allocates aligned objects */
 #define ARCH_KMALLOC_MINALIGN	L1_CACHE_BYTES
 
+#define ARCH_SLAB_MINALIGN	L1_CACHE_BYTES
+
 #define PAGE_UP(addr)	(((addr)+((PAGE_SIZE)-1))&(~((PAGE_SIZE)-1)))
 #define PAGE_DOWN(addr)	((addr)&(~((PAGE_SIZE)-1)))