mm/vma: Change munmap to use vma_munmap_struct() for accounting and surrounding

mm/vma: Change munmap to use vma_munmap_struct() for accounting and surrounding
vmas

Clean up the code by changing the munmap operation to use a structure
for the accounting and munmap variables.

Since remove_mt() is only called in one location and the contents will
be reduced to almost nothing.  The remains of the function can be added
to vms_complete_munmap_vmas().

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>