binder: Use memset_page() in binder_alloc_clear_buf()

The use of kmap() is being deprecated in favor of kmap_local_page()
where it is feasible. With kmap_local_page(), the mapping is per
thread, CPU local and not globally visible.

binder_alloc_clear_buf() is a function where the use of kmap_local_page()
in place of kmap() is correctly suited because the mapping is local to the
thread.

Therefore, use kmap_local_page() / kunmap_local() but, instead of open
coding these two functions and adding a memset() of the virtual address
of the mapping, prefer memset_page().

Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20220425175754.8180-2-fmdefrancesco@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -1175,14 +1175,11 @@ static void binder_alloc_clear_buf(struct binder_alloc *alloc,
 		unsigned long size;
 		struct page *page;
 		pgoff_t pgoff;
-		void *kptr;
 
 		page = binder_alloc_get_page(alloc, buffer,
 					     buffer_offset, &pgoff);
 		size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
-		kptr = kmap(page) + pgoff;
-		memset(kptr, 0, size);
-		kunmap(page);
+		memset_page(page, pgoff, 0, size);
 		bytes -= size;
 		buffer_offset += size;
 	}