CVE-2026-68143: net: slip: serialize receive against buffer reallocation

In the Linux kernel, the following vulnerability has been resolved: net: slip: serialize receive against buffer reallocation sl_realloc_bufs() replaces rbuff and updates buffsize while holding sl->lock. slip_receive_buf() reads those fields and writes through rbuff without holding the lock. An MTU change can therefore race with receive processing. An MTU shrink can expose the new smaller rbuff with the old larger bound, causing an out-of-bounds write. A receive callback which already loaded the old rbuff can instead continue writing after that buffer has been freed. Serialize receive processing with sl_realloc_bufs() by holding sl->lock while consuming each receive batch.