ext4: Use our own write_cache_pages()

ext4: Use our own write_cache_pages()

commit 8e48dcfbd7c0892b4cfd064d682cc4c95a29df32 upstream.

Make a copy of write_cache_pages() for the benefit of
ext4_da_writepages().  This allows us to simplify the code some, and
will allow us to further customize the code in future patches.

There are some nasty hacks in write_cache_pages(), which Linus has
(correctly) characterized as vile.  I've just copied it into
write_cache_pages_da(), without trying to clean those bits up lest I
break something in the ext4's delalloc implementation, which is a bit
fragile right now.  This will allow Dave Chinner to clean up
write_cache_pages() in mm/page-writeback.c, without worrying about
breaking ext4.  Eventually write_cache_pages_da() will go away when I
rewrite ext4's delayed allocation and create a general
ext4_writepages() which is used for all of ext4's writeback.  Until
now this is the lowest risk way to clean up the core
write_cache_pages() function.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Dave Chinner <david@fromorbit.com>
[dev@jaysonking.com: Dropped the hunks which reverted the use of no_nrwrite_index_update, since those lines weren't ever created on 2.6.27.y]
[dev@jaysonking.com: Copied from 2.6.27.y's version of write_cache_pages(), plus the changes to it from patch "vfs: Add no_nrwrite_index_update writeback control flag"]
Signed-off-by: Jayson R. King <dev@jaysonking.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>