The "i--" needs to happen at the start of the loop or it will try to
release something bogus (probably it will crash) and it won't release
the first ->vaddr_page[].

Fixes: 7b4eeed1 ('[media] vmalloc_sg: make sure all pages in vmalloc area are really DMA-ready')
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>