ceph: correct ceph_mds_cap_item field name

The issue_seq is sent with bulk cap releases, not the current sequence
number. See also ceph.git commit 655cddb7c9f3 ("include/ceph_fs: correct
ceph_mds_cap_item field name").

Link: https://tracker.ceph.com/issues/66704


Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>