mptcp: consolidate transition to TCP_CLOSE in mptcp_do_fastclose()

The MPTCP code always set the msk state to TCP_CLOSE before
calling performing the fast-close. Move such state transition in
mptcp_do_fastclose() to avoid some code duplication.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>