Btrfs: use commit root when loading free space cache
    
    A user reported that booting his box up with btrfs root on 3.4 was way
    slower than on 3.3 because I removed the ideal caching code.  It turns out
    that we don't load the free space cache if we're in a commit for deadlock
    reasons, but since we're reading the cache and it hasn't changed yet we are
    safe reading the inode and free space item from the commit root, so do that
    and remove all of the deadlock checks so we don't unnecessarily skip loading
    the free space cache.  The user reported this fixed the slowness.  Thanks,
    
    Tested-by: Calvin Walton <calvin.walton@kepstin.ca>
    Signed-off-by: Josef Bacik <josef@redhat.com>
    Signed-off-by: Chris Mason <chris.mason@oracle.com>