dma-buf: heaps: Clear CMA highages using helper

dma-buf: heaps: Clear CMA highages using helper

Currently the CMA allocator clears highmem pages using
kmap()->clear_page()->kunmap(), but there is a helper
static inline in <linux/highmem.h> that does the same for
us so use clear_highpage() instead of open coding this.

Suggested-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260310-cma-heap-clear-pages-v2-2-ecbbed3d7e6d@kernel.org