Btrfs sometimes needs to flush dirty pages on a bunch of dirty inodes in
order to reclaim metadata reservations.  Unfortunately most helpers in
this area are too smart for us:

1) The normal filemap_fdata* helpers only take range and sync modes, and
   don't give any indication of how much was written, so we can only
   flush full inodes, which isn't what we want in most cases.
2) The normal writeback path requires us to have the s_umount sem held,
   but we can't unconditionally take it in this path because we could
   deadlock.
3) The normal writeback path also skips inodes with I_SYNC set if we
   write with WB_SYNC_NONE.  This isn't the behavior we want under heavy
   ENOSPC pressure, we want to actually make sure the pages are under
   writeback before returning, and if another thread is in the middle of
   writing the file we may return before they're under writeback and
   miss our ordered extents and not properly wait for completion.
4) sync_inode() uses the normal writeback path and has the same problem
   as #3.

What we really want is to call do_writepages() with our wbc.  This way
we can make sure that writeback is actually started on the pages, and we
can control how many pages are written as a whole as we write many
inodes using the same wbc.  Accomplish this with a new helper that does
just that so we can use it for our ENOSPC flushing infrastructure.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>