Revert "ceph: make members in struct ceph_mds_request_args_ext a union"

Revert "ceph: make members in struct ceph_mds_request_args_ext a union"

This reverts commit 3af5ae22030cb59fab4fba35f5a2b62f47e14df9.

ceph_mds_request_args_ext was already (and remains to be) a union.  An
additional anonymous union inside is bogus:

    union ceph_mds_request_args_ext {
        union {
            union ceph_mds_request_args old;
            struct { ... } __attribute__ ((packed)) setattr_ext;
        };
    }

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>