iohelper: use saferead if later write with O_DIRECT
    
    One of the usecases of iohelper is to read from pipe and write
    to file with O_DIRECT. As we read from pipe we can have partial
    read and then we fail to write this data because output file
    is open with O_DIRECT and buffer size is not aligned.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>