slob: No need to zero mapping since it is no longer in use

Reviewed-by: NJoonsoo Kim <js1304@gmail.com>
Acked-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>

mm/slob.c

@@ -97,7 +97,6 @@ typedef struct slob_block slob_t;
 static inline void free_slob_page(struct page *sp)
 {
 	reset_page_mapcount(sp);
-	sp->mapping = NULL;
 }
 
 /*