fs: Allow statmount() in foreign mount namespace

This patch makes use of the new mnt_ns_id field in struct mnt_id_req to
allow users to stat mount entries not in their mount namespace.  The
rules are the same as listmount(), the user must have CAP_SYS_ADMIN in
their user namespace and the target mount namespace must be a child of
the current namespace.

Co-developed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/52a2e17e50ba7aa420bc8bae1d9e88ff593395c1.1719243756.git.josef@toxicpanda.com


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