fanotify: notify on mount attach and detach

Add notifications for attaching and detaching mounts.  The following new
event masks are added:

  FAN_MNT_ATTACH  - Mount was attached
  FAN_MNT_DETACH  - Mount was detached

If a mount is moved, then the event is reported with (FAN_MNT_ATTACH |
FAN_MNT_DETACH).

These events add an info record of type FAN_EVENT_INFO_TYPE_MNT containing
these fields identifying the affected mounts:

  __u64 mnt_id    - the ID of the mount (see statmount(2))

FAN_REPORT_MNT must be supplied to fanotify_init() to receive these events
and no other type of event can be received with this report type.

Marks are added with FAN_MARK_MNTNS, which records the mount namespace from
an nsfs file (e.g. /proc/self/ns/mnt).

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250129165803.72138-3-mszeredi@redhat.com


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