Merge patch series "fsmount: add FSMOUNT_NAMESPACE"

Christian Brauner <brauner@kernel.org> says:

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.

This accompanies OPEN_TREE_NAMESPACE and avoids a needless detour via
OPEN_TREE_NAMESPACE to get the same effect. Will be especially useful
when you mount an actual filesystem to be used as the container rootfs.

* patches from https://patch.msgid.link/20260122-work-fsmount-namespace-v1-0-5ef0a886e646@kernel.org:
  selftests/open_tree_ns: fix compilation
  selftests: add FSMOUNT_NAMESPACE tests
  selftests/statmount: add statmount_alloc() helper
  tools: update mount.h header
  mount: add FSMOUNT_NAMESPACE
  mount: simplify __do_loopback()
  mount: start iterating from start of rbtree

Link: https://patch.msgid.link/20260122-work-fsmount-namespace-v1-0-5ef0a886e646@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>