Btrfs: move definition of the function btrfs_find_new_delalloc_bytes

Move the definition of the function btrfs_find_new_delalloc_bytes() closer
to the function btrfs_dirty_pages(), because in a future commit it will be
used exclusively by btrfs_dirty_pages(). This just moves the function's
definition, with no functional changes at all.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>