ceph: correct ceph_mds_cap_peer field name

The peer seq is used as the issue_seq. Use that name for consistency.
See also ceph.git commit 1da6ef237fc7 ("include/ceph_fs: correct
ceph_mds_cap_peer 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>