mptcp: move subflow cleanup in mptcp_destroy_common()

In the Linux kernel, the following vulnerability has been resolved:

mptcp: move subflow cleanup in mptcp_destroy_common()

If the mptcp socket creation fails due to a CGROUP_INET_SOCK_CREATE

eBPF program, the MPTCP protocol ends-up leaking all the subflows:

the related cleanup happens in __mptcp_destroy_sock() that is not

invoked in such code path.

Address the issue moving the subflow sockets cleanup in the

mptcp_destroy_common() helper, which is invoked in every msk cleanup

path.

Additionally get rid of the intermediate list_splice_init step, which

is an unneeded relic from the past.

The issue is present since before the reported root cause commit, but

any attempt to backport the fix before that hash will require a complete

rewrite.