nommu: pass through vma iterator to shrink_vma()

Rename the function to vmi_shrink_vma() indicate it takes the vma
iterator.  Use the iterator to preallocate and drop the delete function. 
The maple tree is able to do the modification easier than the linked list
and rbtree, so just clear the necessary area in the tree.

add_vma_to_mm() is no longer used, so drop this function.

vmi_add_vma_to_mm() is now only used once, so inline this function into
do_mmap().

Link: https://lkml.kernel.org/r/20230120162650.984577-29-Liam.Howlett@oracle.com


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