FreeBSD: fallback to /boot/ to look for zpool.cache

Up until now zpool.cache has always lived in /boot on FreeBSD.
For the sake of compatibility fallback to /boot if zpool.cache
isn't found in /etc/zfs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10720

module/zfs/spa_config.c

@@ -100,6 +100,10 @@ spa_config_load(void)
 
 	err = zfs_file_open(pathname, O_RDONLY, 0, &fp);
 
+#ifdef __FreeBSD__
+	if (err)
+		err = zfs_file_open(ZPOOL_CACHE_BOOT, O_RDONLY, 0, &fp);
+#endif
 	kmem_free(pathname, MAXPATHLEN);
 
 	if (err)