hugetlb: fix NULL pointer dereference in trace_hugetlbfs_alloc_inode

hugetlb_file_setup() will pass a NULL @dir to hugetlbfs_get_inode(), so we
will access a NULL pointer for @dir.  Fix it and set __entry->dr to 0 if
@dir is NULL.  Because ->i_ino cannot be 0 (see get_next_ino()), there is
no confusing if user sees a 0 inode number.

Link: https://lkml.kernel.org/r/20250106033118.4640-1-songmuchun@bytedance.com
Fixes: 318580a ("hugetlbfs: support tracepoint")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reported-by: Cheung Wall <zzqq0103.hey@gmail.com>
Closes: https://lore.kernel.org/linux-mm/02858D60-43C1-4863-A84F-3C76A8AF1F15@linux.dev/T/#
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Cc: cheung wall <zzqq0103.hey@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: songmuchun

include/trace/events/hugetlbfs.h

@@ -23,7 +23,7 @@ TRACE_EVENT(hugetlbfs_alloc_inode,
 	TP_fast_assign(
 		__entry->dev		= inode->i_sb->s_dev;
 		__entry->ino		= inode->i_ino;
-		__entry->dir		= dir->i_ino;
+		__entry->dir		= dir ? dir->i_ino : 0;
 		__entry->mode		= mode;
 	),