rk_super_dirty() to ext4_handle_dirty_super()

The last user of ext4_mark_super_dirty() in ext4_file_open() is so
rare it can well be modifying the superblock properly by journalling
the change.  Change it and get rid of ext4_mark_super_dirty() as it's
not needed anymore.

Artem: small amendments.
Artem: tested using xfstests for both journalled and non-journalled ext4.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
-Ñ