mptcp: setsockopt: handle SOL_SOCKET in one place only

Move the pre-check to the function that handles all SOL_SOCKET values.

At this point there is complete coverage for all values that were
accepted by the pre-check.

BUSYPOLL functions are accepted but will not have any functionality
yet until its clear how the expected mptcp behaviour should look like.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>