After commit 3f34cfae ("netfilter: on sockopt() acquire sock lock
only in the required scope"), the caller of nf_{get/set}sockopt() must
not hold any lock, but, in such changeset, I forgot to cope with DECnet.

This commit addresses the issue moving the nf call outside the lock,
in the dn_{get,set}sockopt() with the same schema currently used by
ipv4 and ipv6. Also moves the unhandled sockopts of the end of the main
switch statements, to improve code readability.
Reported-by: Petr Vandrovec <petr@vandrovec.name>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=198791#c2
Fixes: 3f34cfae ("netfilter: on sockopt() acquire sock lock only in the required scope")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>