fs: relax permissions for statmount()

It is sufficient to have capabilities in the owning user namespace of
the mount namespace to stat a mount regardless of whether it's reachable
or not.

Link: https://lore.kernel.org/r/bf5961d71ec479ba85806766b0d8d96043e67bba.1719243756.git.josef@toxicpanda.com


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