reboot: refactor and comment the cpu selection code

Small improvements to the code, without changing the way it works:

 - use a local variable, to avoid a small time lapse where reboot_cpu
   can have an invalid value

 - comment the code which is not easy to understand at a glance

 - merge two identical code blocks into one

 - replace pointer arithmetics with equivalent array syntax

Link: https://lkml.kernel.org/r/20201103214025.116799-4-mcroce@linux.microsoft.comSigned-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/reboot.c

@@ -553,20 +553,24 @@ static int __init reboot_setup(char *str)
 			break;
 
 		case 's':
-			if (isdigit(*(str+1)))
-				reboot_cpu = simple_strtoul(str+1, NULL, 0);
-			else if (str[1] == 'm' && str[2] == 'p' &&
-							isdigit(*(str+3)))
-				reboot_cpu = simple_strtoul(str+3, NULL, 0);
-			else
+			/*
+			 * reboot_cpu is s[mp]#### with #### being the processor
+			 * to be used for rebooting. Skip 's' or 'smp' prefix.
+			 */
+			str += str[1] == 'm' && str[2] == 'p' ? 3 : 1;
+
+			if (isdigit(str[0])) {
+				int cpu = simple_strtoul(str, NULL, 0);
+
+				if (cpu >= num_possible_cpus()) {
+					pr_err("Ignoring the CPU number in reboot= option. "
+					"CPU %d exceeds possible cpu number %d\n",
+					cpu, num_possible_cpus());
+					break;
+				}
+				reboot_cpu = cpu;
+			} else
 				*mode = REBOOT_SOFT;
-			if (reboot_cpu >= num_possible_cpus()) {
-				pr_err("Ignoring the CPU number in reboot= option. "
-				       "CPU %d exceeds possible cpu number %d\n",
-				       reboot_cpu, num_possible_cpus());
-				reboot_cpu = 0;
-				break;
-			}
 			break;
 
 		case 'g':