netfilter: ipv6: nf_defrag: fix leakage of unqueued fragments

With commit 997dd964 ("net: IP6 defrag: use rbtrees in
nf_conntrack_reasm.c"), nf_ct_frag6_reasm() is now called from
nf_ct_frag6_queue(). With this change, nf_ct_frag6_queue() can fail
after the skb has been added to the fragment queue and
nf_ct_frag6_gather() was adapted to handle this case.

But nf_ct_frag6_queue() can still fail before the fragment has been
queued. nf_ct_frag6_gather() can't handle this case anymore, because it
has no way to know if nf_ct_frag6_queue() queued the fragment before
failing. If it didn't, the skb is lost as the error code is overwritten
with -EINPROGRESS.

Fix this by setting -EINPROGRESS directly in nf_ct_frag6_queue(), so
that nf_ct_frag6_gather() can propagate the error as is.

Fixes: 997dd964 ("net: IP6 defrag: use rbtrees in nf_conntrack_reasm.c")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>