staging/lustre/osc: fixed a typo in osc_cache_truncate_start

staging/lustre/osc: fixed a typo in osc_cache_truncate_start

Dump osc_extent waiting instead of ext in case error occurred.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3359
Lustre-change: http://review.whamcloud.com/6402
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>