lib/test_bitmap.c: make use of EXP2_IN_BITS

Commit 30544ed5 ("lib/bitmap: introduce bitmap_replace() helper")
introduced some new test cases to the test_bitmap.c module.  Among these
it also introduced an (unused) definition.  Let's make use of
EXP2_IN_BITS.
Reported-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: http://lkml.kernel.org/r/20200121151847.75223-1-andriy.shevchenko@linux.intel.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/test_bitmap.c

@@ -278,6 +278,8 @@ static void __init test_replace(void)
 	unsigned int nlongs = DIV_ROUND_UP(nbits, BITS_PER_LONG);
 	DECLARE_BITMAP(bmap, 1024);
 
+	BUILD_BUG_ON(EXP2_IN_BITS < nbits * 2);
+
 	bitmap_zero(bmap, 1024);
 	bitmap_replace(bmap, &exp2[0 * nlongs], &exp2[1 * nlongs], exp2_to_exp3_mask, nbits);
 	expect_eq_bitmap(bmap, exp3_0_1, nbits);