mptcp: rename mptcp_setsockopt_sol_ip_set_transparent()

Next patch extends this function so that it's not specific to
IP_TRANSPARENT. Change function name to mptcp_setsockopt_sol_ip_set().

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Maxim Galaganov <max@internet.ru>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>