ext2/4: use is_power_of_2()

Replace n & (n - 1) with is_power_of_2(n)

Signed-off-by: vignesh babu <vignesh.babu@wipro.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>