ceph: make members in struct ceph_mds_request_args_ext a union

In ceph mainline it will allow to set the btime in the setattr request
and just add a 'btime' member in the union 'ceph_mds_request_args' and
then bump up the header version to 4. That means the total size of union
'ceph_mds_request_args' will increase sizeof(struct ceph_timespec) bytes,
but in kclient it will increase the sizeof(setattr_ext) bytes for each
request.

Since the MDS will always depend on the header's vesion and front_len
members to decode the 'ceph_mds_request_head' struct, at the same time
kclient hasn't supported the 'btime' feature yet in setattr request,
so it's safe to do this change here.

This will save 48 bytes memories for each request.

Fixes: 4f1ddb1e

 ("ceph: implement updated ceph_mds_request_head structure")
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>