ext4: Enable the multiblock allocator by default

Enable the multiblock allocator by default.

Fix ext4_show_options() so if it is not enabled, the nomballoc option
included in /proc/mounts.

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>