ext3's fsync/fdatasync implementation is currently syncing the inode via a
full journal commit even if it was unaltered.

Fix that up by exporting the core VFS's inode sync function to modules and
calling it if the inode is dirty.  We need to do it this way so that the
inode is moved to the appropriate superblock list and so that the i_state
dirty flags are appropriately updated.

This speeds up ext3 fsync() for file overwrites by a factor of four (disk
non-writeback) to forty (disk in writeback mode).