Staticize kstat_default_update()

This is only used via ->ks_update of `kstat_t *`.
This isn't exported nor do headers have its prototype.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #686

module/spl/spl-kstat.c

@@ -327,7 +327,7 @@ restart:
 	return (-rc);
 }
 
-int
+static int
 kstat_default_update(kstat_t *ksp, int rw)
 {
 	ASSERT(ksp != NULL);