lib: add find_nth{,_and,_andnot}_bit()

Kernel lacks for a function that searches for Nth bit in a bitmap.
Usually people do it like this:
	for_each_set_bit(bit, mask, size)
		if (n-- == 0)
			return bit;

We can do it more efficiently, if we:
1. find a word containing Nth bit, using hweight(); and
2. find the bit, using a helper fns(), that works similarly to
   __ffs() and ffz().

fns() is implemented as a simple loop. For x86_64, there's PDEP instruction
to do that: ret = clz(pdep(1 << idx, num)). However, for large bitmaps the
most of improvement comes from using hweight(), so I kept fns() simple.

New find_nth_bit() is ~70 times faster on x86_64/kvm in find_bit benchmark:
find_nth_bit:                  7154190 ns,  16411 iterations
for_each_bit:                505493126 ns,  16315 iterations

With all that, a family of 3 new functions is added, and used where
appropriate in the following patches.

Signed-off-by: Yury Norov <yury.norov@gmail.com>

include/linux/bitops.h

@@ -247,6 +247,25 @@ static inline unsigned long __ffs64(u64 word)
 	return __ffs((unsigned long)word);
 }
 
+/**
+ * fns - find N'th set bit in a word
+ * @word: The word to search
+ * @n: Bit to find
+ */
+static inline unsigned long fns(unsigned long word, unsigned int n)
+{
+	unsigned int bit;
+
+	while (word) {
+		bit = __ffs(word);
+		if (n-- == 0)
+			return bit;
+		__clear_bit(bit, &word);
+	}
+
+	return BITS_PER_LONG;
+}
+
 /**
  * assign_bit - Assign value to a bit in memory
  * @nr: the bit to set