NTFS: Change ntfs_map_runlist_nolock() to only decompress the mapping pairs

      if the requested vcn is inside it.  Otherwise we get into problems
      when we try to map an out of bounds vcn because we then try to map
      the already mapped runlist fragment which causes
      ntfs_mapping_pairs_decompress() to fail and return error.  Update
      ntfs_attr_find_vcn_nolock() accordingly.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>