btrfs: Remove chunk_objectid parameter of btrfs_alloc_dev_extent

THe function is always called with chunk_objectid set to
BTRFS_FIRST_CHUNK_TREE_OBJECTID. Let's collapse the parameter in the
function itself. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>