ipv6: fix a race in ip6_sock_set_v6only()

ipv6: fix a race in ip6_sock_set_v6only()

It is unlikely that this function will be ever called
with isk->inet_num being not zero.

Perform the check on isk->inet_num inside the locked section
for complete safety.

Fixes: 9b115749acb24 ("ipv6: add ip6_sock_set_v6only")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260216102202.3343588-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>