fs: export an inode_update_time helper

fs: export an inode_update_time helper

commit e60feb445fce9e51c1558a6aa7faf9dd5ded533b upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>