ocfs2/trivial: Wrap ocfs2_sysfile_cluster_lock_key within define.

Actually ocfs2_sysfile_cluster_lock_key is only used if we enable
CONFIG_DEBUG_LOCK_ALLOC. Wrap it so that we can avoid a building
warning.
fs/ocfs2/sysfile.c:53: warning: ‘ocfs2_sysfile_cluster_lock_key’
defined but not used

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>