staging:zram:xvmalloc.c Fix a typo.

Not exactly sure if this is a typo or not, due to my search
results comming up with not that many hits. Either its dereferenceable
or dereferencable from the two I choose the later. if it's wrong let me know
and I'll resend.
Signed-off-by: NJustin P. Mattock <justinmattock@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/zram/xvmalloc.c

@@ -46,7 +46,7 @@ static void clear_flag(struct block_header *block, enum blockflags flag)
 }
 
 /*
- * Given <page, offset> pair, provide a derefrencable pointer.
+ * Given <page, offset> pair, provide a dereferencable pointer.
  * This is called from xv_malloc/xv_free path, so it
  * needs to be fast.
  */