plugin: enable multiple plugins for the same hook

CRIU provides two plugins for checkpoint/restore of GPU applications:
amdgpu and cuda. Both plugins use the `RESUME_DEVICES_LATE` hook to
enable restore:

    CR_PLUGIN_REGISTER_HOOK(CR_PLUGIN_HOOK__RESUME_DEVICES_LATE, amdgpu_plugin_resume_devices_late)
    CR_PLUGIN_REGISTER_HOOK(CR_PLUGIN_HOOK__RESUME_DEVICES_LATE, cuda_plugin_resume_devices_late)

However, CRIU currently does not support running more than one plugin
for the same hook. As a result, when both plugins are installed, the
resume function for CUDA applications is not executed. To fix this,
we need to make sure that both `plugin_resume_devices_late()` functions
return `-ENOTSUP` when restore is not supported.

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

plugins/amdgpu/amdgpu_plugin.c

@@ -1809,7 +1809,7 @@ int amdgpu_plugin_resume_devices_late(int target_pid)
 	fd = open(AMDGPU_KFD_DEVICE, O_RDWR | O_CLOEXEC);
 	if (fd < 0) {
 		pr_perror("failed to open kfd in plugin");
-		return -1;
+		return -ENOTSUP;
 	}
 
 	args.pid = target_pid;
@@ -1818,6 +1818,7 @@ int amdgpu_plugin_resume_devices_late(int target_pid)
 	if (kmtIoctl(fd, AMDKFD_IOC_CRIU_OP, &args) == -1) {
 		if (errno == ESRCH) {
 			pr_info("Pid %d has no kfd process info\n", target_pid);
+			exit_code = -ENOTSUP;
 		} else {
 			pr_perror("restore late ioctl failed");
 			exit_code = -1;

plugins/cuda/cuda_plugin.c

@@ -408,7 +408,7 @@ int resume_device(int pid, int checkpointed)
 int cuda_plugin_resume_devices_late(int pid)
 {
 	if (plugin_disabled) {
-		return 0;
+		return -ENOTSUP;
 	}
 
 	return resume_device(pid, 1);