mm: change munmap splitting order and move_vma()

Splitting can be more efficient when the order is not of concern.  Change
do_vmi_align_munmap() to reduce walking of the tree during split
operations.

move_vma() must also be altered to remove the dependency of keeping the
original VMA as the active part of the split.  Transition to using vma
iterator to look up the prev and/or next vma after munmap.

[Liam.Howlett@oracle.com: fix vma iterator initialization]
  Link: https://lkml.kernel.org/r/20230126212011.980350-1-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20230120162650.984577-39-Liam.Howlett@oracle.com


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