mmap: convert vma_link() vma iterator

Avoid using the maple tree interface directly.

Link: https://lkml.kernel.org/r/20230120162650.984577-11-Liam.Howlett@oracle.com


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