ext4: move sysfs code from super.c to fs/ext4/sysfs.c

Also statically allocate the ext4_kset and ext4_feat objects, since we
only need exactly one of each, and it's simpler and less code if we
drop the dynamic allocation and deallocation when it's not needed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>