dropping the read lock and taking the write lock we were not checking
        whether someone else did not already do the work we wanted to do.
      - Rename ntfs_find_vcn_nolock() to ntfs_attr_find_vcn_nolock().
      - Tidy up some comments in fs/ntfs/runlist.c.
      - Add LCN_ENOMEM and LCN_EIO definitions to fs/ntfs/runlist.h.
Signed-off-by: NAnton Altaparmakov <aia21@cantab.net>