mm: add temporary vma iterator versions of vma_merge(), split_vma(), and __split_vma()

These wrappers are short-lived in this patch set so that each user can be
converted on its own.  In the end, these functions are renamed in one
commit.

Link: https://lkml.kernel.org/r/20230120162650.984577-15-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>