CVE-2026-72122: can: bcm: fix lockless bound/ifindex race and silent RX_SETUP failure

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

can: bcm: fix lockless bound/ifindex race and silent RXSETUP failure

bcmsendmsg() reads bo->ifindex and checks bo->bound before taking locksock(), while bcmnotify(), bcmconnect() and bcmrelease() all mutate both fields under that same lock. Because the lockless reads and the locked writes are unordered with respect to each other, a racing bcmnotify() (device unregister) or bcmconnect() (concurrent bind on another thread sharing the socket) can make bcmsendmsg() observe an inconsistent combination, e.g. a stale bound=1 together with the now-cleared ifindex=0, silently turning a socket bound to a specific CAN interface into one that also matches "any" interface.

Keep the lockless bo->bound check purely as a fast-path reject, and move the ifindex read (and a bo->bound re-check) into the locked section, where every writer already serializes. This removes the possibility of observing the two fields torn against each other, rather than trying to fix it with more READONCE()/WRITEONCE() pairs on two independently updated fields. Annotate the now-purely-lockless bo->bound accesses consistently across all its write sites.

Also fix bcmrxsetup() silently returning success when the target device disappears concurrently instead of reporting -ENODEV, so a broken RX op is no longer left registered as if it had succeeded.