btrfs: make prealloc_file_extent_cluster take btrfs_inode

The vfs inode is only used for a pair of inode_lock/unlock calls all
other uses call for btrfs_inode.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>