[PATCH] Fix synchronous writers to wait properly for the result

[PATCH] Fix synchronous writers to wait properly for the result

Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> points out a bug in
ll_rw_block() usage.

Typical usage is:

	mark_buffer_dirty(bh);
	ll_rw_block(WRITE, 1, &bh);
	wait_on_buffer(bh);

the problem is that if the buffer was locked on entry to this code sequence
(due to in-progress I/O), ll_rw_block() will not wait, and start new I/O.  So
this code will wait on the _old_ I/O, and will then continue execution,
leaving the buffer dirty.

It turns out that all callers were only writing one buffer, and they were all
waiting on that writeout.  So I added a new sync_dirty_buffer() function:

	void sync_dirty_buffer(struct buffer_head *bh)
	{
		lock_buffer(bh);
		if (test_clear_buffer_dirty(bh)) {
			get_bh(bh);
			bh->b_end_io = end_buffer_io_sync;
			submit_bh(WRITE, bh);
		} else {
			unlock_buffer(bh);
		}
	}

which allowed a fair amount of code to be removed, while adding the desired
data-integrity guarantees.

UFS has its own wrappers around ll_rw_block() which got in the way, so this
operation was open-coded in that case.