ll_rw_block audit

[Christoph Hellwig <hch@lst.de>]

I've just sent a patch to fsdevel to remove the SWRITE* flags and add
a new helper to write buffers using proper locking, which tricked me
into an audit of all ll_rw_block users.  There's not a lot left now,
so it might be time to kill them.  For writes we only have 6 users left:

fs/buffer.c:		ll_rw_block(WRITE, 1, &bh);
fs/hfs/hfs_fs.h:	ll_rw_block(WRITE, 1, &bh);
fs/reiserfs/journal.c:	ll_rw_block(WRITE, 1, &bh);
fs/reiserfs/journal.c:	ll_rw_block(WRITE, 1, &tbh);

plus two odd commented out ones in jbd/jbd2:

fs/jbd/recovery.c:			/* ll_rw_block(WRITE, 1, &nbh); */
fs/jbd2/recovery.c:			/* ll_rw_block(WRITE, 1, &nbh); */

The one in hfs is obviously a broken attempt to implement
sync_dirty_buffer, I'll send a patch to fix it.

That leaves:

  - write_boundary_block.

	This one is only used by __mpage_writepage and is a data
	integrity operation if WB_SYNC_ALL is set.  It should probably
	use write_dirty_buffer at least for that case.  Is it worth
	keeping the trylock for WB_SYNC_NONE writes?
	Btw, what's the reason this function is in buffer.c instead
	of mpage.c?

 - write_ordered_buffers and flush_commit_list in reiserfs.  Both of
   these look like data integrity operations to me, so using
   write_dirty_buffer seems in order.

For the read side we have 38 users. Most of them wait on the buffer
in some more or less broken form.  These should be replaced with the
bh_uptodate_or_lock/bh_submit_read combination, or possibly a helper
combining the two.  There's also a lot of places that look like
broken reimplementations of __bread_slow.