pidfs: check for valid pid namespace

When we access a no-current task's pid namespace we need check that the
task hasn't been reaped in the meantime and it's pid namespace isn't
accessible anymore.

The user namespace is fine because it is only released when the last
reference to struct task_struct is put and exit_creds() is called.

Link: https://lore.kernel.org/r/20240926-klebt-altgedienten-0415ad4d273c@brauner


Fixes: 5b08bd40 ("pidfs: allow retrieval of namespace file descriptors")
CC: stable@vger.kernel.org # v6.11
Signed-off-by: Christian Brauner <brauner@kernel.org>