exportfs: add flag to allow marking export operations as only supporting file handles

Hey,

Some filesystems like kernfs and pidfs support file handles as a
convenience to enable the use of name_to_handle_at(2) and
open_by_handle_at(2) but don't want to and cannot be reliably exported.
Add a flag that allows them to mark their export operations accordingly
and make NFS check for its presence.

@Amir, I'll reorder the patches such that this series comes prior to the
pidfs file handle series. Doing it that way will mean that there's never
a state where pidfs supports file handles while also being exportable.
It's probably not a big deal but it's definitely cleaner. It also means
the last patch in this series to mark pidfs as non-exportable can be
dropped. Instead pidfs export operations will be marked as
non-exportable in the patch that they are added in.

Thanks!
Christian

To: Amir Goldstein <amir73il@gmail.com>
To: Jeff Layton <jlayton@kernel.org>
Cc: Erin Shepherd <erin.shepherd@e43.eu>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-nfs@vger.kernel.org

---
Changes in v2:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v1: https://lore.kernel.org/r/20241201-work-exportfs-v1-0-b850dda4502a@kernel.org



--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 2,
    "change-id": "20241201-work-exportfs-cd49bee773c5",
    "prefixes": [],
    "base-branch": "vfs-6.14.pidfs",
    "history": {
      "v1": [
        "20241201-work-exportfs-v1-0-b850dda4502a@kernel.org"
      ]
    }
  }
}