btrfs: use inode already stored in local variable at btrfs_rmdir()

There's no need to call d_inode(dentry) when calling btrfs_unlink_inode()
since we have already stored that in a local inode variable. So just use
the local variable to make the code less verbose.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>