ext4: do not mark superblock as dirty unnecessarily

Commit a0375156 cleaned up superblock
dirtying handling, but missed one place. This patch does what was
intended: if we have the journal, then we update the superblock
through the journal rather than doing this directly.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>