mmap: convert vma_expand() to use vma iterator

Use the vma iterator instead of the maple state for type safety and for
consistency through the mm code.

Link: https://lkml.kernel.org/r/20230120162650.984577-14-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>