bitmap: replace _reg_op(REG_OP_ISFREE) with find_next_bit()

_reg_op(REG_OP_ISFREE) can be trivially replaced with find_next_bit().
Doing that opens room for potential small_const_nbits() optimization.

CC: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Yury Norov <yury.norov@gmail.com>

lib/bitmap.c

@@ -840,7 +840,7 @@ int bitmap_allocate_region(unsigned long *bitmap, unsigned int pos, int order)
 {
 	unsigned int len = BIT(order);
 
-	if (!__reg_op(bitmap, pos, order, REG_OP_ISFREE))
+	if (find_next_bit(bitmap, pos + len, pos) < pos + len)
 		return -EBUSY;
 	bitmap_set(bitmap, pos, len);
 	return 0;