btrfs: remove the cross file system checks from remap

The sb check is already done in do_clone_file_range, and the mnt check
(which will hopefully go away in a subsequent patch) is done in
ioctl_file_clone().  Remove the check in our code and put an ASSERT() to
make sure it doesn't change underneath us.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/reflink.c

@@ -781,9 +781,7 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 		if (btrfs_root_readonly(root_out))
 			return -EROFS;
 
-		if (file_in->f_path.mnt != file_out->f_path.mnt ||
-		    inode_in->i_sb != inode_out->i_sb)
-			return -EXDEV;
+		ASSERT(inode_in->i_sb == inode_out->i_sb);
 	}
 
 	/* Don't make the dst file partly checksummed */