f2fs: Replace expression with offsetof()

Use the existing offsetof() macro instead of duplicating code.
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -2696,7 +2696,7 @@ int f2fs_recover_inode_page(struct f2fs_sb_info *sbi, struct page *page)
 	src = F2FS_INODE(page);
 	dst = F2FS_INODE(ipage);
 
-	memcpy(dst, src, (unsigned long)&src->i_ext - (unsigned long)src);
+	memcpy(dst, src, offsetof(struct f2fs_inode, i_ext));
 	dst->i_size = 0;
 	dst->i_blocks = cpu_to_le64(1);
 	dst->i_links = cpu_to_le32(1);