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

include/sys/spa_impl.h

@@ -447,6 +447,7 @@ extern void spa_event_post(sysevent_t *ev);
 extern int param_set_deadman_failmode_common(const char *val);
 extern void spa_set_deadman_synctime(hrtime_t ns);
 extern void spa_set_deadman_ziotime(hrtime_t ns);
+extern const char *spa_history_zone(void);
 
 #ifdef	__cplusplus
 }