return 0 after parsing "use_session_token" arg (#1388)

return 0 after parsing "use_session_token" arg

I hope this might fix #651.

There's clearly an open bug in #651 confirmed by several users related to the parsing of the `-o use_session_token`. Looking at the source, I noticed that there was a return value everywhere except here, so I suspect this may be responsible.

src/s3fs.cpp

@@ -4285,6 +4285,7 @@ static int my_fuse_opt_proc(void* data, const char* arg, int key, struct fuse_ar
         }
         if (0 == STR2NCMP(arg, "use_session_token")) {
             is_use_session_token = true;
+            return 0;
         }
         if(0 == STR2NCMP(arg, "ibm_iam_endpoint=")){
             std::string endpoint_url;