LU-3843 clio: add debug message in osc_completion()

Replace LASSERT with LASSERTF in osc_completion, thus we can get
more info when the LASSERT is triggered.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I1b365050d5c2348b4cdc83f2ea71f36cce7f9456