fs: start all kthreads in nullfs

Point init_task's fs_struct (root and pwd) at a private nullfs instance
instead of the mutable rootfs. All kthreads now start isolated in nullfs
and must use scoped_with_init_fs() for any path resolution.

PID 1 is moved from nullfs into the initramfs by init_userspace_fs().
Usermodehelper threads use userspace_init_fs via the umh flag in
copy_fs(). All subsystems that need init's filesystem state for path
resolution already use scoped_with_init_fs() from earlier commits in
this series.

This isolates kthreads from userspace filesystem state and makes it
hard to perform filesystem operations from kthread context.

Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-23-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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