mm/mempolicy: fix mbind_range() arguments to vma_merge()

Fuzzing produced an invalid argument to vma_merge() which was caught by
the newly added verification of the number of VMAs being removed on
process exit.  Analyzing the failure eventually resulted in finding an
issue with the search of a VMA that started at address 0, which caused an
underflow and thus the loss of many VMAs being tracked in the tree.  Fix
the underflow by changing the search of the maple tree to use the start
address directly.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 66850be55e8e ("mm/mempolicy: use vma iterator & maple state instead of vma linked list")
Signed-off-by: Liam R. Howlett <[email protected]>
Reported-by: kernel test robot <[email protected]>
  Link: https://lore.kernel.org/r/[email protected]
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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