[media] v4l: vb2-dma-contig: align buffer size to PAGE_SIZE

Most operations on DMA and DMABUF framework require the size of a buffer to be
page aligned. This patch guarantees the requirement is satisfied for all
vb2-dma-contig buffers.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>