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.c

@@ -26,6 +26,7 @@
 #include <linux/radix-tree.h>
 #include <linux/vmalloc.h>
 #include <linux/string.h>
+#include <linux/compat.h>
 
 #include "send.h"
 #include "backref.h"
@@ -6371,13 +6372,12 @@ static void btrfs_root_dec_send_in_progress(struct btrfs_root* root)
 	spin_unlock(&root->root_item_lock);
 }
 
-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)
 {
 	int ret = 0;
 	struct btrfs_root *send_root = BTRFS_I(file_inode(mnt_file))->root;
 	struct btrfs_fs_info *fs_info = send_root->fs_info;
 	struct btrfs_root *clone_root;
-	struct btrfs_ioctl_send_args *arg = NULL;
 	struct btrfs_key key;
 	struct send_ctx *sctx = NULL;
 	u32 i;
@@ -6413,13 +6413,6 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 		goto out;
 	}
 
-	arg = memdup_user(arg_, sizeof(*arg));
-	if (IS_ERR(arg)) {
-		ret = PTR_ERR(arg);
-		arg = NULL;
-		goto out;
-	}
-
 	/*
 	 * Check that we don't overflow at later allocations, we request
 	 * clone_sources_count + 1 items, and compare to unsigned long inside
@@ -6660,7 +6653,6 @@ out:
 	if (sctx && !IS_ERR_OR_NULL(sctx->parent_root))
 		btrfs_root_dec_send_in_progress(sctx->parent_root);
 
-	kfree(arg);
 	kvfree(clone_sources_tmp);
 
 	if (sctx) {