ext4: Use is_power_of_2()

Replace (n & (n-1)) in the context of power of 2 checks with
is_power_of_2()

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: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>