Possible fix for bug #146711
    If a withdraw is called before we've a chance to relock the trans
    lock, the sd_log_head points to the wrong place, and a umount will
    fail on asserts because of this.
    Adding one puts sd_log_head at a value that passes the assert.  The
    value may not be correct for on disk, but we've withdrawn so there is
    no more disk io.
    If we're not withdrawn, the next io will grab the trans lock, which
    will fill sd_log_head with the correct value.