landlock: Require LANDLOCK_ACCESS_FS_MAKE_REG for whiteout creation

In the Linux kernel, the following vulnerability has been resolved:

landlock: Require LANDLOCK_ACCESS_FS_MAKE_REG for whiteout creation

Whiteout objects are used in the upper layer of an OverlayFS to
indicate that the file with this name does not exist in the unified
view, even if it is present in one of the lower layer file systems.

For the userspace implementations of OverlayFS (fuse-overlayfs),
whiteout objects can be created from userspace as well:

* mknod(2) with S_IFCHR and makedev(0, 0)
* renameat2(2) with RENAME_WHITEOUT,
creating the whiteout in the old place of the moved file.

This commit guards whiteout creation in both of these cases with
LANDLOCK_ACCESS_FS_MAKE_REG. Whiteout objects are *not* considered
character devices and are not bound to a driver.

LANDLOCK_ACCESS_FS_MAKE_REG describes the same permission class as a
whiteout object: creating one is the only S_IFCHR creation that the VFS
exempts from CAP_MKNOD, so it is as unprivileged as creating a regular
file, while LANDLOCK_ACCESS_FS_MAKE_CHAR and
LANDLOCK_ACCESS_FS_MAKE_BLOCK keep meaning the creation of devices that
expose a kernel interface [1].

For the mknod(2) case, introduce a Landlock erratum. The creation of
whiteout objects through mknod(2) was previously guarded using
LANDLOCK_ACCESS_FS_MAKE_CHAR, and it is now guarded using
LANDLOCK_ACCESS_FS_MAKE_REG.

For the renameat2(2) case, fix a bug: Before this commit, renameat2(2)
with RENAME_WHITEOUT would create a directory entry even when all
LANDLOCK_ACCESS_FS_MAKE_* rights were denied.

This does not affect normal renames within layered OverlayFS mounts:
When doing a regular rename() on a mounted fuse-overlayfs, it is the
fuse-overlayfs daemon that exercises renameat2() with RENAME_WHITEOUT,
and only the Landlock domain of that daemon is checked there.

Depends-on: 49c9e09d9610 ("landlock: Fix handling of disconnected directories")
Depends-on: fe72ce6710cb ("landlock: Add errata documentation section")
[mic: Record why LANDLOCK_ACCESS_FS_MAKE_REG is the matching right, and
add link(2) to the user doc]