Return 0 for process gpu mem if no entry found

If no entry is found with the process PID in the gpu_mem_total BPF map,
return 0 for the process's GPU memory usage.

Test: libmeminfo_test
Bug: 208594690
Change-Id: If22b754b3f9a570e0fc12741385e4b461f52e0db
1 file changed