media: videobuf2-core: Prevent size alignment wrapping buffer size to 0

PAGE_ALIGN() may wrap the buffer size around to 0. Prevent this by
checking that the aligned value is not smaller than the unaligned one.

Note on backporting to stable: the file used to be under
drivers/media/v4l2-core, it was moved to the current location after 4.14.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>