btrfs: fix send ioctl on 32bit with 64bit kernel

We pass in a pointer in our send arg struct, this means the struct size
doesn't match with 32bit user space and 64bit kernel space.  Fix this by
adding a compat mode and doing the appropriate conversion.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ move structure to the beginning, next to receive 32bit compat ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.h

@@ -130,5 +130,5 @@ enum {
 #define BTRFS_SEND_A_MAX (__BTRFS_SEND_A_MAX - 1)
 
 #ifdef __KERNEL__
-long btrfs_ioctl_send(struct file *mnt_file, void __user *arg);
+long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg);
 #endif