statmount: accept fd as a parameter

statmount: accept fd as a parameter

Extend `struct mnt_id_req` to take in a fd and introduce STATMOUNT_BY_FD
flag. When a valid fd is provided and STATMOUNT_BY_FD is set, statmount
will return mountinfo about the mount the fd is on.

This even works for "unmounted" mounts (mounts that have been umounted
using umount2(mnt, MNT_DETACH)), if you have access to a file descriptor
on that mount. These "umounted" mounts will have no mountpoint and no
valid mount namespace. Hence, we unset the STATMOUNT_MNT_POINT and
STATMOUNT_MNT_NS_ID in statmount.mask for "unmounted" mounts.

In case of STATMOUNT_BY_FD, given that we already have access to an fd
on the mount, accessing mount information without a capability check
seems fine because of the following reasons:

- All fs related information is available via fstatfs() without any
  capability check.
- Mount information is also available via /proc/pid/mountinfo (without
  any capability check).
- Given that we have access to a fd on the mount which tells us that we
  had access to the mount at some point (or someone that had access gave
  us the fd). So, we should be able to access mount info.

Co-developed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>
Link: https://patch.msgid.link/20251129091455.757724-3-b.sachdev1904@gmail.com
Acked-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>