keeping the mft mirror in sync with the mft when mirrored mft records
      are written.  The functions are write_mft_record{,_nolock}().  The
      implementation is quite rudimentary for now with lots of things not
      implemented yet but I am not sure any of them can actually occur so
      I will wait for people to hit each one and only then implement it.
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>