[pcp] proc/interrupts parsing fix for large CPU counts

[Arthur Kepner <akepner@xxxxxxx>]

The linux pmda assumes that /proc/interrupts contains lines formatted 
like so:

123:   1    2   3   .....

i.e., the interrupt number is precisely 3 characters wide. It's 
usually 4 characters wide on larger systems.

This fixes it.

--- 
diff --git a/src/pmdas/linux/proc_interrupts.c 
b/src/pmdas/linux/proc_interrupts.c
index 63045c2..5b13a0b 100644
--- a/src/pmdas/linux/proc_interrupts.c
+++ b/src/pmdas/linux/proc_interrupts.c
@@ -73,9 +73,9 @@ refresh_proc_interrupts(proc_interrupts_t *proc_interrupts)
     }
 
     while (fgets(buf, sizeof(buf), fp) != NULL) {
-       if (buf[3] != ':')
+       s = strchr(buf, ':');
+       if (s == NULL)
            continue;
-       s = buf + 3;
        is_syscall = 0;
 
        if (sscanf(buf, "%u:", &intr) != 1) {