fs: add fchroot()

Add a file descriptor based counterpart to chroot(2). This has been
overdue for a long time. It is the natural companion to fchdir() and
avoids re-resolving a path that the caller already holds a file
descriptor to. No TOCTOU between resolving the target and changing the
root. It composes with modern fd-based APIs meaning it works with O_PATH
file descriptors and file descriptors to detached mount trees created
via open_tree(OPEN_TREE_CLONE).

The permission model is identical to chroot(2). The caller must have
CAP_SYS_CHROOT in its user namespace, must pass MAY_EXEC | MAY_CHDIR
permission checks on the target directory, and LSMs are consulted via
the same security_path_chroot() hook.

The system call takes a flags argument for future extensibility which
must currently be zero.

Link: https://patch.msgid.link/20260724-work-failfs-v2-3-485dabbae185@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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