mm/migrate: remove cpages-- in migrate_vma_finalize()

The variable struct migrate_vma->cpages is only used in
migrate_vma_setup().  There is no need to decrement it in
migrate_vma_finalize() since it is never checked.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20200827190735.12752-1-rcampbell@nvidia.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>