MFC r264494:
  Use SMB_QUERY_FS_SIZE_INFO request to populate statfs structure.
  When server doesn't support this request, try to use SMB_INFO_ALLOCATION.
  And use SMB_COM_QUERY_INFORMATION_DISK request as fallback.

MFC r264600:
  Remove redundant unlock.

  This code was removed from the opensolaris and darwin's
  netsmb implementations, in DfBSD it also has been disabled.