lib/bitmap: extend comment for bitmap_(from,to)_arr32()

On LE systems bitmaps are naturally ordered, therefore we can potentially
use bitmap_copy routines when converting from 32-bit arrays, even if host
system is 64-bit. But it may lead to out-of-bond access due to unsafe
typecast, and the bitmap_(from,to)_arr32 comment doesn't explain that
clearly

CC: Alexander Gordeev <[email protected]>
CC: Andy Shevchenko <[email protected]>
CC: Christian Borntraeger <[email protected]>
CC: Claudio Imbrenda <[email protected]>
CC: David Hildenbrand <[email protected]>
CC: Heiko Carstens <[email protected]>
CC: Janosch Frank <[email protected]>
CC: Rasmus Villemoes <[email protected]>
CC: Sven Schnelle <[email protected]>
CC: Vasily Gorbik <[email protected]>
Signed-off-by: Yury Norov <[email protected]>

include/linux/bitmap.h

@@ -264,8 +264,12 @@ static inline void bitmap_copy_clear_tail(unsigned long *dst,
 }
 
 /*
- * On 32-bit systems bitmaps are represented as u32 arrays internally, and
- * therefore conversion is not needed when copying data from/to arrays of u32.
+ * On 32-bit systems bitmaps are represented as u32 arrays internally. On LE64
+ * machines the order of hi and lo parts of numbers match the bitmap structure.
+ * In both cases conversion is not needed when copying data from/to arrays of
+ * u32. But in LE64 case, typecast in bitmap_copy_clear_tail() may lead
+ * to out-of-bound access. To avoid that, both LE and BE variants of 64-bit
+ * architectures are not using bitmap_copy_clear_tail().
  */
 #if BITS_PER_LONG == 64
 void bitmap_from_arr32(unsigned long *bitmap, const u32 *buf,