ext4: Enable extents by default

Turn on extents feature by default in ext4 filesystem, to get wider
testing of extents feature in ext4dev.  This can be disabled using 
-o noextents.  

Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>