pstore: Introduce get_reason_str() to pstore

Recently, there has been some changes in kmsg_dump() below and they have been applied to linus-tree.
 (1) kmsg_dump(KMSG_DUMP_KEXEC) was removed.
     http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=a3dd3323058d281abd584b15ad4c5b65064d7a61

 (2) The order of "enum kmsg_dump_reason" was modified.
     http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=c22ab332902333f83766017478c1ef6607ace681

Replace the fragile reason_str array with a more robust solution that
will not be broken by future re-arrangements of the enum values.

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Link: https://lkml.org/lkml/2012/3/16/417
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/platform.c

@@ -68,9 +68,25 @@ void pstore_set_kmsg_bytes(int bytes)
 /* Tag each group of saved records with a sequence number */
 static int	oopscount;
 
-static char *reason_str[] = {
-	"Oops", "Panic", "Kexec", "Restart", "Halt", "Poweroff", "Emergency"
-};
+static const char *get_reason_str(enum kmsg_dump_reason reason)
+{
+	switch (reason) {
+	case KMSG_DUMP_PANIC:
+		return "Panic";
+	case KMSG_DUMP_OOPS:
+		return "Oops";
+	case KMSG_DUMP_EMERG:
+		return "Emergency";
+	case KMSG_DUMP_RESTART:
+		return "Restart";
+	case KMSG_DUMP_HALT:
+		return "Halt";
+	case KMSG_DUMP_POWEROFF:
+		return "Poweroff";
+	default:
+		return "Unknown";
+	}
+}
 
 /*
  * callback from kmsg_dump. (s2,l2) has the most recently
@@ -85,17 +101,15 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	unsigned long	s1_start, s2_start;
 	unsigned long	l1_cpy, l2_cpy;
 	unsigned long	size, total = 0;
-	char		*dst, *why;
+	char		*dst;
+	const char	*why;
 	u64		id;
 	int		hsize, ret;
 	unsigned int	part = 1;
 	unsigned long	flags = 0;
 	int		is_locked = 0;
 
-	if (reason < ARRAY_SIZE(reason_str))
-		why = reason_str[reason];
-	else
-		why = "Unknown";
+	why = get_reason_str(reason);
 
 	if (in_nmi()) {
 		is_locked = spin_trylock(&psinfo->buf_lock);