netlink: specs: mptcp: rename the MPTCP path management spec

We assume in handful of places that the name of the spec is
the same as the name of the family. We could fix that but
it seems like a fair assumption to make. Rename the MPTCP
spec instead.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>