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");
+}