btrfs: drop argument tree from submit_extent_page

Now that we're sure the tree from argument is same as the one we can get
from the page's inode io_tree, drop the redundant argument.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>