V4L/DVB (4685): Fix compiler warning in drivers/media/video/video-buf.c

Using a double cast to avoid compiler warnings when building for PAE. 
Compiler doesn't like direct casting of a 32 bit ptr to 64 bit integer.

Signed-off-by: Martin J. Bligh <mbligh@google.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>