btrfs: cleanup, use kmalloc_array/kcalloc array helpers

Convert kmalloc(nr * size, ..) to kmalloc_array that does additional
overflow checks, the zeroing variant is kcalloc.

Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/inode.c

@@ -463,7 +463,7 @@ again:
 	 */
 	if (inode_need_compress(inode)) {
 		WARN_ON(pages);
-		pages = kzalloc(sizeof(struct page *) * nr_pages, GFP_NOFS);
+		pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
 		if (!pages) {
 			/* just bail out to the uncompressed code */
 			goto cont;