fs: export an inode_update_time helper

If you already have an inode and need to update the time on the inode
there is no way to do this properly.  Export this helper to allow file
systems to update time on the inode so the appropriate handler is
called, either ->update_time or generic_update_time.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>