V4L/DVB (7023): Fix a regresion left by changeset 7e65d6e8f6df

Changeset 7e65d6e8f6df removed a very bad hack on mmap(). However, the fixes
weren't considering usermap and overlay memory models. This were breaking
direct reading from /dev/video?, used mostly by mpeg aware drivers.

Thanks to Steven Toth <stoth@linuxtv.org> for reporting the issue and
bissecting it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>