pf: set scope in pf_refragment6()

Link-local traffic needs to have a scope embedded before it's passed on
to ip6_output(). Do so in pf_refragment6(), because when we end up here
in the output path we may have passed through ip6_output() already
(before being reassembled), where the scope would have been removed.

Re-embed the scope so that link-local traffic is sent correctly.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39062