mm/mmap: Don't use __vma_adjust() in shift_arg_pages()

mm/mmap: Don't use __vma_adjust() in shift_arg_pages()

Introduce shrink_vma() which uses the vma_prepare() and vma_complete()
functions to reduce the vma coverage.

Convert shift_arg_pages() to use expand_vma() and the new shrink_vma()
function.  Remove support from __vma_adjust() to reduce a vma size since
shift_arg_pages() is the only user that shrinks a VMA in this way.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>