dma-buf: heaps: Clear CMA pages with clear_pages()

dma-buf: heaps: Clear CMA pages with clear_pages()

As of commit 62a9f5a85b98
"mm: introduce clear_pages() and clear_user_pages()" we can
clear a range of pages with a potentially assembly-optimized
call.

Instead of using a memset, use this helper to clear the whole
range of pages from the CMA allocation.

Reviewed-by: T.J. Mercier <tjmercier@google.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-v2-1-ecbbed3d7e6d@kernel.org