[PATCH] Add a sync_fs super_block operation

This is infrastructure for fixing the journalled-data ext3 unmount data
loss problem. It was sent for comment to linux-fsdevel a week ago; there
was none.

Add a `sync_fs' superblock operation whose mandate is to perform
filesystem-specific operations to ensure a successful sync.

It is called in two places:

1: fsync_super() - for umount.

2: sys_sync() - for global sync.

In the sys_sync() case we call all the ->write_super() methods first.
write_super() is an async flushing operation.  It should not block.

After that, we call all the ->sync_fs functions.  This is independent
of the state of s_dirt!  That was all confused up before, and in this
patch ->write_super() and ->sync_fs() are quite separate.

With ext3 as an example, the initial ->write_super() will start a
transaction, but will not wait on it.  (But only if s_dirt was set!)

The first ->sync_fs() call will get the IO underway.

The second ->sync_fs() call will wait on the IO.

And we really do need to be this elaborate, because all the testing of
s_dirt in there makes ->write_super() an unreliable way of detecting
when the VFS is trying to sync the filesystem.

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