ext4: replace ktype default_attrs with default_groups

The kobj_type default_attrs field is being replaced by the
default_groups field. Replace the default_attrs field in ext4_sb_ktype
and ext4_feat_ktype with default_groups. Use the ATTRIBUTE_GROUPS macro
to create ext4_groups and ext4_feat_groups.

Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>