kernel/dma: remove unnecessary unmap_kernel_range

vunmap will remove ptes.

Link: https://lkml.kernel.org/r/20210322021806.892164-3-npiggin@gmail.com


Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Cédric Le Goater <clg@kaod.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>