HAMMER - Fix root inode creation for slave.

HAMMER - Fix root inode creation for slave.

* The root inode for a PFS is only supposed to be created for PFS masters.
  PFS slaves get their root inode from the mirroring operation.  No longer
  create the root inode for a PFS slave.

  The root inode was being incorrectly created for slaves as well as
  masters, causing the root inode from the mirroring operation to be
  ignored.

  This caused the root directory for a slave to show up as empty when
  doing a mirror-copy from a HAMMER version 1 filesystem to a HAMMER
  version 2 filesystem.

Reported-by:  "Simon 'corecode' Schubert" <corecode@fs.ei.tum.de>