LU-1274 osc: Do not grab mutex of cl_lock for glimpse

LU-1274 osc: Do not grab mutex of cl_lock for glimpse

Otherwise this will cause client eviction if that lock is being
flushed and OST happens to be slow to finish the IO.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I8cbd9c38a317f4afaf3b0e237efe55e4c0f991c2
Reviewed-on: http://review.whamcloud.com/2426
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>