From: Chris Mason <mason@suse.com>

There's a small window where the filesystem can be unmounted during
writeback_inodes.  The end result is the iput done by sync_sb_inodes could
be done after the FS put_super and and the super has been removed from all
lists.

The fix is to hold the s_umount sem during sync_sb_inodes to make sure
the FS doesn't get unmounted.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>