kernfs: use namespace id instead of pointer for hashing and comparison

kernfs: use namespace id instead of pointer for hashing and comparison

kernfs uses the namespace tag as both a hash seed (via init_name_hash())
and a comparison key in the rbtree. The resulting hash values are exposed
to userspace through directory seek positions (ctx->pos), and the raw
pointer comparisons in kernfs_name_compare() encode kernel pointer
ordering into the rbtree layout.

This constitutes a KASLR information leak since the hash and ordering
derived from kernel pointers can be observed from userspace.

Fix this by using the 64-bit namespace id (ns_common::ns_id) instead of
the raw pointer value for both hashing and comparison. The namespace id
is a stable, non-secret identifier that is already exposed to userspace
through other interfaces (e.g., /proc/pid/ns/, ioctl NS_GET_NSID).

Introduce kernfs_ns_id() as a helper that extracts the namespace id from
a potentially-NULL ns_common pointer, returning 0 for the no-namespace
case.

All namespace equality checks in the directory iteration and dentry
revalidation paths are also switched from pointer comparison to ns_id
comparison for consistency.

Signed-off-by: Christian Brauner <brauner@kernel.org>