All of the relocation code avoids using the cached state, despite
everywhere using the normal

  lock_extent()
  // do something
  unlock_extent()

pattern.  Fix this by plumbing a cached state throughout all of these
functions in order to allow for less tree searches.
Reviewed-by: Filipe Manana <fdmanana@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>