fs: add an ioctl to get the mnt ns id from nsfs

In order to utilize the listmount() and statmount() extensions that
allow us to call them on different namespaces we need a way to get the
mnt namespace id from user space.  Add an ioctl to nsfs that will allow
us to extract the mnt namespace id in order to make these new extensions
usable.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/180449959d5a756af7306d6bda55f41b9d53e3cb.1719243756.git.josef@toxicpanda.com


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