Abstract away platform specific superblock references

The zfsvfs->z_sb field is Linux specified and should be abstracted.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9697

module/os/linux/zfs/zfs_ioctl_os.c

@@ -65,6 +65,12 @@
 #include <linux/miscdevice.h>
 #include <linux/slab.h>
 
+boolean_t
+zfs_vfs_held(zfsvfs_t *zfsvfs)
+{
+	return (zfsvfs->z_sb != NULL);
+}
+
 int
 zfs_vfs_ref(zfsvfs_t **zfvp)
 {
@@ -75,6 +81,12 @@ zfs_vfs_ref(zfsvfs_t **zfvp)
 	return (0);
 }
 
+void
+zfs_vfs_rele(zfsvfs_t *zfsvfs)
+{
+	deactivate_super(zfsvfs->z_sb);
+}
+
 static int
 zfsdev_state_init(struct file *filp)
 {