migration/multifd: Cleanup TLS iochannel referencing

migration/multifd: Cleanup TLS iochannel referencing

Commit a1af605bd5 ("migration/multifd: fix hangup with TLS-Multifd due to
blocking handshake") introduced a thread for TLS channels, which will
resolve the issue on blocking the main thread.  However in the same commit
p->c is slightly abused just to be able to pass over the pointer "p" into
the thread.

That's the major reason we'll need to conditionally free the io channel in
the fault paths.

To clean it up, using a separate structure to pass over both "p" and "tioc"
in the tls handshake thread.  Then we can make it a rule that p->c will
never be set until the channel is completely setup.  With that, we can drop
the tricky conditional unref of the io channel in the error path.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240222095301.171137-2-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>