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/[e-mail]/

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

fs/hugetlbfs/inode.c

···
 	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;