vfs: Call ->sync_fs() even if s_dirt is 0 (version 4)

sync_filesystems() has a condition that if wait == 0 and s_dirt == 0, then
->sync_fs() isn't called. This does not really make much sence since s_dirt is
generally used by a filesystem to mean that ->write_super() needs to be called.
But ->sync_fs() does different things. I even suspect that some filesystems
(btrfs?) sets s_dirt just to fool this logic.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>