mount: add FSMOUNT_NAMESPACE

mount: add FSMOUNT_NAMESPACE

Add FSMOUNT_NAMESPACE flag to fsmount() that creates a new mount
namespace with the newly created filesystem attached to a copy of the
real rootfs. This returns a namespace file descriptor instead of an
O_PATH mount fd, similar to how OPEN_TREE_NAMESPACE works for open_tree().

This allows creating a new filesystem and immediately placing it in a
new mount namespace in a single operation, which is useful for container
runtimes and other namespace-based isolation mechanisms.

The rootfs mount is created before copying the real rootfs for the new
namespace meaning that the mount namespace id for the mount of the root
of the namespace is bigger than the child mounted on top of it. We've
never explicitly given the guarantee for such ordering and I doubt
anyone relies on it. Accepting that lets us avoid copying the mount
again and also avoids having to massage may_copy_tree() to grant an
exception for fsmount->mnt->mnt_ns being NULL.

Link: https://patch.msgid.link/20260122-work-fsmount-namespace-v1-3-5ef0a886e646@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>