V4L/DVB: videobuf: rename videobuf_mmap_free and add sanity checks

This function is not specific to mmap, hence the rename.
Add a check whether we are not streaming or reading (for read mode that
uses the stream queue) before freeing anything.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>