Make struct vdev_disk_t be platform private

Linux defines different vdev_disk_t members to macOS, but they are
only used in vdev_disk.c so move the declaration there.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #10452

module/os/linux/zfs/vdev_disk.c

@@ -37,6 +37,11 @@
 #include <linux/msdos_fs.h>
 #include <linux/vfs_compat.h>
 
+typedef struct vdev_disk {
+	struct block_device		*vd_bdev;
+	krwlock_t			vd_lock;
+} vdev_disk_t;
+
 /*
  * Unique identifier for the exclusive vdev holder.
  */