selftests/filesystems: add failfs selftests

Test the failfs semantics and both new entry points:

- fchdir(FD_FAILFS_ROOT):

* working directory lookups and getcwd() fail
* other sentinels are r

selftests/filesystems: add failfs selftests

Test the failfs semantics and both new entry points:

- fchdir(FD_FAILFS_ROOT):

* working directory lookups and getcwd() fail
* other sentinels are rejected
* the state is recoverable while the root is untouched

- fchroot() with regular fds:
* chroot parity
* CAP_SYS_CHROOT required
* ENOTDIR/EBADF/EINVAL checks

- fchroot(FD_FAILFS_ROOT):

* absolute lookups, stat, statfs and opens of the root including O_PATH fail with EOPNOTSUPP
* dirfd-anchored I/O keeps working
* ".." walks clamp at the top of the mount tree
* /proc magic links resolve but can't be stat through
* absolute symlinks fail while relative symlinks keep resolving

- Unprivileged entry requires no_new_privs and is rejected for
chrooted callers and for a shared fs_struct

- entering makes the task count as chrooted so user namespace creation
fails

- Nothing can be mounted on top of failfs and OPEN_TREE_CLONE is
rejected; the overmount test runs in a private mount namespace so a
regression cannot touch the host root

- setns() to a kept mount namespace fd restores root and working
directory

- The failfs root is inherited across fork() and absolute exec fails

- Exec by fd of a dynamically linked binary fails on opening its
absolute PT_INTERP interpreter

The exec tests run the exec in a child so a wrongly successful exec
cannot replace the test image and masquerade as a pass.

Link: https://patch.msgid.link/20260724-work-failfs-v2-6-485dabbae185@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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