lib: bitmap: add missing mask in bitmap_shift_right

There is no guarantee that *src does not contain garbage bits outside
the lower nbits, so we need to mask it before the shift-and-assign.
Signed-off-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/bitmap.h

@@ -284,7 +284,7 @@ static inline void bitmap_shift_right(unsigned long *dst,
 			const unsigned long *src, int n, int nbits)
 {
 	if (small_const_nbits(nbits))
-		*dst = *src >> n;
+		*dst = (*src & BITMAP_LAST_WORD_MASK(nbits)) >> n;
 	else
 		__bitmap_shift_right(dst, src, n, nbits);
 }