kdb: Fix off by one error in kdb_cpu()

There was a follow on replacement patch against the prior
"kgdb: Timeout if secondary CPUs ignore the roundup".

See: https://lkml.org/lkml/2015/1/7/442

This patch is the delta vs the patch that was committed upstream:
  * Fix an off-by-one error in kdb_cpu().
  * Replace NR_CPUS with CONFIG_NR_CPUS to tell checkpatch that we
    really want a static limit.
  * Removed the "KGDB: " prefix from the pr_crit() in debug_core.c
    (kgdb-next contains a patch which introduced pr_fmt() to this file
    to the tag will now be applied automatically).

Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

kernel/debug/debug_core.c

@@ -604,7 +604,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 		   online_cpus)
 		cpu_relax();
 	if (!time_left)
-		pr_crit("KGDB: Timed out waiting for secondary CPUs.\n");
+		pr_crit("Timed out waiting for secondary CPUs.\n");
 
 	/*
 	 * At this point the primary processor is completely

kernel/debug/kdb/kdb_main.c

@@ -2256,7 +2256,7 @@ static int kdb_cpu(int argc, const char **argv)
 	/*
 	 * Validate cpunum
 	 */
-	if ((cpunum > NR_CPUS) || !kgdb_info[cpunum].enter_kgdb)
+	if ((cpunum >= CONFIG_NR_CPUS) || !kgdb_info[cpunum].enter_kgdb)
 		return KDB_BADCPUNUM;
 
 	dbg_switch_cpu = cpunum;