jbd: unexport journal_update_superblock

Remove the unused EXPORT_SYMBOL(journal_update_superblock).

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>