Restrict kstats and print real pointers

There are several places where we use zfs_dbgmsg and %p to
print pointers. In the Linux kernel, these values obfuscated
to prevent information leaks which means the pointers aren't
very useful for debugging crash dumps. We decided to restrict
the permissions of dbgmsg (and some other kstats while we were
at it) and print pointers with %px in zfs_dbgmsg as well as
spl_dumpstack

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: sara hartse <sara.hartse@delphix.com>
Closes #8467 
Closes #8476

lib/libzpool/kernel.c

@@ -437,6 +437,7 @@ seq_printf(struct seq_file *m, const char *fmt, ...)
 void
 procfs_list_install(const char *module,
     const char *name,
+    mode_t mode,
     procfs_list_t *procfs_list,
     int (*show)(struct seq_file *f, void *p),
     int (*show_header)(struct seq_file *f),