mds: add ceph.dir.subvolume get vxattr

mds: add ceph.dir.subvolume get vxattr

This introduces handling for the "ceph.dir.subvolume" virtual xattr on directories.
- Returns ASCII "1" when the directory is a subvolume root, "0" otherwise.

QA:
- Extend CephFS subvolume tests to validate vxattr retrieval and behavior around setting/removal.

Fixes: https://tracker.ceph.com/issues/72556
Signed-off-by: Edwin Rodriguez <edwin.rodriguez1@ibm.com>
(cherry picked from commit 51fb4b6abe9daba651150055c7f768d8a6d61d48)

 Conflicts:
    qa/tasks/cephfs/test_subvolume.py
    - resolved conflicts of https://github.com/ceph/ceph/commit/3d5dd1235316775a9dbfdd225c76da09efe02e31
      not backported to tentacle yet.