hugetlbfs: prevent filesystem stacking of hugetlbfs

syzbot found issues with having hugetlbfs on a union/overlay as reported
in [1].  Due to the limitations (no write) and special functionality of
hugetlbfs, it does not work well in filesystem stacking.  There are no
know use cases for hugetlbfs stacking.  Rather than making modifications
to get hugetlbfs working in such environments, simply prevent stacking.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Reported-by: [email protected]
Suggested-by: Amir Goldstein <[email protected]>
Signed-off-by: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Miklos Szeredi <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Colin Walters <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

fs/hugetlbfs/inode.c

@@ -1364,6 +1364,12 @@ hugetlbfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_magic = HUGETLBFS_MAGIC;
 	sb->s_op = &hugetlbfs_ops;
 	sb->s_time_gran = 1;
+
+	/*
+	 * Due to the special and limited functionality of hugetlbfs, it does
+	 * not work well as a stacking filesystem.
+	 */
+	sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
 	sb->s_root = d_make_root(hugetlbfs_get_root(sb, ctx));
 	if (!sb->s_root)
 		goto out_free;