fs: don't try and remove empty rbtree node

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fs: don't try and remove empty rbtree node When copying a namespace we won't have added the new copy into the namespace rbtree until after the copy succeeded. Calling free_mnt_ns() will try to remove the copy from the rbtree which is invalid. Simply free the namespace skeleton directly.

References

• https://git.kernel.org/stable/c/a8b155a2c30dc9a5ba837aa5fcba9a47cc031a9b
• https://git.kernel.org/stable/c/229fd15908fe1f99b1de4cde3326e62d1e892611