ceph: remove unused variables in ceph_mdsmap_decode()

Fix build warnings:

  fs/ceph/mdsmap.c: In function ‘ceph_mdsmap_decode’:
  fs/ceph/mdsmap.c:192:7: warning: variable ‘info_cv’ set but not used [-Wunused-but-set-variable]
  fs/ceph/mdsmap.c:177:7: warning: variable ‘state_seq’ set but not used [-Wunused-but-set-variable]
  fs/ceph/mdsmap.c:123:15: warning: variable ‘mdsmap_cv’ set but not used [-Wunused-but-set-variable]

Note that p is increased in ceph_decode_*.

Signed-off-by: Jia Yang <jiayang5@huawei.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>