hfs: get rid of hfs_sync_super

This patch makes hfs stop using the VFS '->write_super()' method along with
the 's_dirt' superblock flag, because they are on their way out.

The whole "superblock write-out" VFS infrastructure is served by the
'sync_supers()' kernel thread, which wakes up every 5 (by default) seconds and
writes out all dirty superblocks using the '->write_super()' call-back.  But the
problem with this thread is that it wastes power by waking up the system every
5 seconds, even if there are no diry superblocks, or there are no client
file-systems which would need this (e.g., btrfs does not use
'->write_super()'). So we want to kill it completely and thus, we need to make
file-systems to stop using the '->write_super()' VFS service, and then remove
it together with the kernel thread.

Tested using fsstress from the LTP project.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/hfs/extent.c b/fs/hfs/extent.c
index 2c16316..a67955a 100644
--- a/fs/hfs/extent.c
+++ b/fs/hfs/extent.c
@@ -432,7 +432,7 @@
 		if (inode->i_ino < HFS_FIRSTUSER_CNID)
 			set_bit(HFS_FLG_ALT_MDB_DIRTY, &HFS_SB(sb)->flags);
 		set_bit(HFS_FLG_MDB_DIRTY, &HFS_SB(sb)->flags);
-		sb->s_dirt = 1;
+		hfs_mark_mdb_dirty(sb);
 	}
 	return res;