mm/ksm: fix race with VMA iteration and mm_struct teardown

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

mm/ksm: fix race with VMA iteration and mm_struct teardown

exit_mmap() will tear down the VMAs and maple tree with the mmap_lock held

in write mode. Ensure that the maple tree is still valid by checking

ksm_test_exit() after taking the mmap_lock in read mode, but before the

for_each_vma() iterator dereferences a destroyed maple tree.

Since the maple tree is destroyed, the flags telling lockdep to check an

external lock has been cleared. Skip the for_each_vma() iterator to avoid

dereferencing a maple tree without the external lock flag, which would

create a lockdep warning.