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/[email protected]
Fixes: 318580a ("hugetlbfs: support tracepoint")
Signed-off-by: Muchun Song <[email protected]>
Reported-by: Cheung Wall <[email protected]>
Closes: https://lore.kernel.org/linux-mm/[email protected]/T/#
Reviewed-by: Hongbo Li <[email protected]>
Cc: cheung wall <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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;
 	),