ip6mr: only set ip6mr_table from setsockopt when ip6mr_new_table succeeds

Currently, raw6_sk(sk)->ip6mr_table is set unconditionally during
ip6_mroute_setsockopt(MRT6_TABLE). A subsequent attempt at the same
setsockopt will fail with -ENOENT, since we haven't actually created
that table.

A similar fix for ipv4 was included in commit 5e1859fb ("ipv4: ipmr:
various fixes and cleanups").

Fixes: d1db275d

 ("ipv6: ip6mr: support multiple tables")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>