f2fs: check if inmem_pages list is empty correctly

`cur' will never be NULL, we should check inmem_pages list instead.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -328,7 +328,7 @@ void drop_inmem_page(struct inode *inode, struct page *page)
 			break;
 	}
 
-	f2fs_bug_on(sbi, !cur || cur->page != page);
+	f2fs_bug_on(sbi, list_empty(head) || cur->page != page);
 	list_del(&cur->list);
 	mutex_unlock(&fi->inmem_lock);