ext4: ext4_find_next_zero_bit needs an aligned address on some arch

ext4_find_next_zero_bit and ext4_find_next_bit needs a long aligned
address on x8_64. Add mb_find_next_zero_bit and mb_find_next_bit
and use them in the mballoc.

Fix: https://bugzilla.redhat.com/show_bug.cgi?id=433286

Eric Sandeen debugged the problem and suggested the fix.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by:      Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>