DEBIAN-CVE-2026-99172: Debian/linux vulnerability

In the Linux kernel, the following vulnerability has been resolved: ipc: drop the mqueue mount with kernunmount() on setup failure mqinitns() creates the mqueue mount with fcmountlongterm(), so its ->mntns is MNTNSINTERNAL. When setting up the mqueue or ipc sysctls or msginitns() fails afterwards, createipcns() drops it with a plain mntput(). With ->mntns set that takes the fast path of mntputnoexpire(). The count drops to zero and nothing ever kills the mount, so the mount and the mqueue superblock are leaked. Commit dc55e35f9e81 ("ipc: Store mqueue sysctls in the ipc namespace") added the first failure path after mqinitns() and never dropped the mount. Commit 7229d74e5e8c ("ipc: createipcns: drop mqueue mount on sysctl setup failure") tried to fix that with a mntput() but that doesn't help for a long-term mount. Use kernunmount(). freeipc() open-codes the same thing for a namespace that made it.