btrfs: remove the dirty_page local variable

btrfs: remove the dirty_page local variable

Inside btrfs_buffered_write(), we have a local variable @dirty_pages,
recording the number of pages we dirtied in the current iteration.

However we do not really need that variable, since it can be calculated
from @pos and @copied.

In fact there is already a problem inside the short copy path, where we
use @dirty_pages to calculate the range we need to release.
But that usage assumes sectorsize == PAGE_SIZE, which is no longer true.

Instead of keeping @dirty_pages and cause incorrect usage, just
calculate the number of dirtied pages inside btrfs_dirty_pages().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>