powerpc/xive: Add warning if target CPU not found

Add a warn_once to warn if the CPU target is not found. This could help
to find about any such usecase.

This is a very rare case, which either means mask was empty or
atomic update failed for all online CPUs. So it is worth printing that
path for potential fix.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Yury Norov <ynorov@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260427044715.559137-5-sshegde@linux.ibm.com

Author: Shrikanth Hegde

arch/powerpc/sysdev/xive/common.c

@@ -564,6 +564,7 @@ static int xive_find_target_in_mask(const struct cpumask *mask,
 			return cpu;
 	}
 
+	WARN_ONCE(1, "target CPU not found in mask: %*pbl\n", cpumask_pr_args(mask));
 	return -1;
 }