CVE-2026-46210: media: iris: fix use-after-free of fmt_src during MBPF check

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

media: iris: fix use-after-free of fmtsrc during MBPF check

During concurrency testing, multiple instances can run in parallel, and each instance uses its own inst->lock while the core->lock protects the list of active instances. The race happens because these locks cover different scopes, inst->lock protects only the internals of a single instance, while the Macro Blocks Per Frame (MBPF) checker walks the core list under core->lock and reads fields like fmtsrc->width and fmtsrc->height. At the same time, irisclose() may free fmtsrc and fmtdst under inst->lock while the instance is still present in the core list. This allows a situation where the MBPF checker, still iterating through the core list, reaches an instance whose fmtsrc was already freed by another thread and ends up dereferencing a dangling pointer, resulting in a use-after-free. This happens because the MBPF checker assumes that any instance in the core list is fully valid, but the freeing of fmtsrc and fmtdst without removing the instance from the core list is not correct.

The correct ordering is to defer freeing fmtsrc and fmtdst until after the instance has been removed from the core list and all teardown under the core lock has completed, ensuring that no dangling pointers are ever exposed during MBPF checks.