CVE-2026-100081: xen-netback: defer xenvif_mcast_addr_list_free()

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

xen-netback: defer xenvifmcastaddrlistfree()

In order for the function to legitimately avoid locking/RCU, it needs to run after all TX/RX processing has finished/stopped: Lookups happen from xenvifstartxmit(), while adds/removes happen from xenviftxaction() (called out of xenvifpoll()).

Move the call out of xenvifdisconnectdata() (the call to the function from connect()'s error path doesn't require any cleaning up there), and put it past the synchronization point in backenddisconnect().

This is XSA-527 / CVE-2026-100081.