Make spa_history_zone platform-dependent in kernel

This function should only return "linux" on Linux.

Move the kernel part of the function out of common code.
Fix the tests for FreeBSD.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10079

module/os/linux/zfs/spa_misc_os.c

@@ -102,3 +102,9 @@ param_set_slop_shift(const char *buf, zfs_kernel_param_t *kp)
 
 	return (0);
 }
+
+const char *
+spa_history_zone(void)
+{
+	return ("linux");
+}

module/zfs/spa_history.c

@@ -180,16 +180,6 @@ spa_history_write(spa_t *spa, void *buf, uint64_t len, spa_history_phys_t *shpp,
 	return (0);
 }
 
-static char *
-spa_history_zone(void)
-{
-#ifdef _KERNEL
-	return ("linux");
-#else
-	return (NULL);
-#endif
-}
-
 /*
  * Post a history sysevent.
  *
@@ -622,6 +612,14 @@ spa_history_log_version(spa_t *spa, const char *operation, dmu_tx_t *tx)
 	    u->nodename, u->release, u->version, u->machine);
 }
 
+#ifndef _KERNEL
+const char *
+spa_history_zone(void)
+{
+	return (NULL);
+}
+#endif
+
 #if defined(_KERNEL)
 EXPORT_SYMBOL(spa_history_create_obj);
 EXPORT_SYMBOL(spa_history_get);