v4l2src: fix v4l2_munmap() for compressed formats

v4l2src: fix v4l2_munmap() for compressed formats

Make sure we always call munmap() with the same size we called mmap()
with before.

Current v4l2src uses the same structure for VIDIOC_QUERYBUF, VIDIOC_QBUF
and v4l2_munmap calls. The problem is that the video buffer size (length)
may vary for compressed or emulated bufs. VIDIOC_QBUF will change it if
we pass the pointer of a v4l2_buffer. This is why we should avoid using
same variable for mmap and video buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=671126