mptcp: do not fill info not used by the PM in used

Only the in-kernel PM uses the number of address and subflow limits
allowed per connection.

It then makes more sense not to display such info when other PMs are
used not to confuse the userspace by showing limits not being used.

While at it, we can get rid of the "val" variable and add indentations
instead.

It would have been good to have done this modification directly in
commit 4d25247d ("mptcp: bypass in-kernel PM restrictions for non-kernel PMs")
but as we change a bit the behaviour, it is fine not to backport it to
stable.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>