Add define to enable autotrim to default to on

In FreeBSD trim has defaulted to on for several
years. In order to minimize POLA violations on
import it's important to maintain this default
when importing vendored openzfs in to FreeBSD
base.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10719

include/sys/spa.h

@@ -733,10 +733,16 @@ typedef enum spa_mode {
  * Send TRIM commands in-line during normal pool operation while deleting.
  *	OFF: no
  *	ON: yes
+ * NB: IN_FREEBSD_BASE is defined within the FreeBSD sources.
  */
 typedef enum {
 	SPA_AUTOTRIM_OFF = 0,	/* default */
-	SPA_AUTOTRIM_ON
+	SPA_AUTOTRIM_ON,
+#ifdef IN_FREEBSD_BASE
+	SPA_AUTOTRIM_DEFAULT = SPA_AUTOTRIM_ON,
+#else
+	SPA_AUTOTRIM_DEFAULT = SPA_AUTOTRIM_OFF,
+#endif
 } spa_autotrim_t;
 
 /*