powerpc/watchpoint/xmon: Don't allow breakpoint overwriting

Xmon allows overwriting breakpoints because it's supported by only
one DAWR. But with multiple DAWRs, overwriting becomes ambiguous
or unnecessary complicated. So let's not allow it.
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Michael Neuling <mikey@neuling.org>
Link: https://lore.kernel.org/r/20200514111741.97993-16-ravi.bangoria@linux.ibm.com

arch/powerpc/xmon/xmon.c

@@ -1399,6 +1399,10 @@ bpt_cmds(void)
 			printf("Hardware data breakpoint not supported on this cpu\n");
 			break;
 		}
+		if (dabr.enabled) {
+			printf("Couldn't find free breakpoint register\n");
+			break;
+		}
 		mode = 7;
 		cmd = inchar();
 		if (cmd == 'r')