Some applications (xawtv, qv4l2) mix read and mmap calls. Allow switching from read mode back to mmap mode (by doing a reqbufs). Signed-off-by: Hans de Goede <hdegoede@redhat.com> Acked-by: Jean-Francois Moine <moinejf@free.fr> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>