The patch fixes a few problems in the writer throttling code.  Mainly
in the situation where a single large file is being written out.

That file could be parked on sb->locked_inodes due to pdflush
writeback, and the writer throttling path coming out of
balance_dirty_pages() forgot to look for inodes on ->locked_inodes.

The net effect was that the amount of dirty memory was exceeding the
limit set in /proc/sys/vm/dirty_async_ratio, possibly to the point
where the system gets seriously choked.

The patch removes sb->locked_inodes altogether and teaches the
throttling code to look for inodes on sb->s_io as well as sb->s_dirty.

Also, just leave unwritten dirty pages on mapping->io_pages, and
unwritten dirty inodes on sb->s_io.  Putting them back onto
->dirty_pages and ->dirty_inodes was fairly pointless, given that both
lists need to be looked at.