fs: relax permissions for listmount()

It is sufficient to have capabilities in the owning user namespace of
the mount namespace to list all mounts regardless of whether they are
reachable or not.

Link: https://lore.kernel.org/r/8adc0d3f4f7495faacc6a7c63095961f7f1637c7.1719243756.git.josef@toxicpanda.com


Signed-off-by: Christian Brauner <brauner@kernel.org>