btrfs: restore BTRFS_SEQ_LAST when looking up qgroup backref lookup

In the patch a2c8d27e ("btrfs: use a structure to pass arguments to
backref walking functions") Filipe converted everybody to using a new
context struct to use for backref lookups, but accidentally dropped the
BTRFS_SEQ_LAST usage that exists for qgroups.  Add this back so we have
the previous behavior.

Fixes: a2c8d27e ("btrfs: use a structure to pass arguments to backref walking functions")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>