Andreas Beckmann gave me a report that nilfs logged the following
warnings when it got a disk full:

  nilfs_sufile_do_cancel_free: segment 0 must be clean
  nilfs_sufile_do_cancel_free: segment 1 must be clean

These arise from a duplicate call to nilfs_segctor_cancel_freev in an
error path of log writer.  This will fix the issue.
Reported-by: Andreas Beckmann <debian@abeckmann.de>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>