test/librados/c_write_operations.cc: free ressources

Release completion as soon as no longer needed.

Fix for:

CID 1219593 (#1 of 1): Resource leak (RESOURCE_LEAK)
 leaked_storage: Variable completion going out of scope
 leaks the storage it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>