ext2: Move ext2_write_super() out of ext2_setup_super()

Move ext2_write_super() out of ext2_setup_super() as a preparation for the
next patch that adds a new lock for superblock fields.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>