LU-2737 osc: double unlock in osc_lru_shrink()

It could unlock twice in osc_lru_shrink() if the cl_io_init() failed.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Idf3ebafced7f7d3a68c13ea20d12cf221e9d74ce