dma-buf: Drop dma_buf_k(un)map

It's unused.

10 years ago, back when 32bit was still fairly common and trying to
not exhaust vmalloc space sounded like a worthwhile goal, adding these
to dma_buf made sense.

Reality is that they simply never caught on, and nowadays everyone who
needs plenty of buffers will run in 64bit mode anyway.

Also update the docs in this area to adjust them to reality.

The actual hooks in dma_buf_ops will be removed once all the
implementations are gone.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-10-daniel.vetter@ffwll.ch