fsck.gfs2: Revise "undo" processing
    
    This patch has to do with "undo" processing. This is where pass1 has
    processed a bunch of data blocks before encountering an error. If the
    error is "fatal" processing is stopped immediately and the work done
    to that point (up to the fatal error block) is backed out. If the
    error is not fatal, processing may continue for the rest of the
    indirect block it's processing. The problem occurs when it finds a
    non-fatal error block, then later it decides it cannot continue. In
    that case, the "undo" function needs to back out changes, but it
    needs to finish working through the data blocks on the rest of that
    indirect block. This patch allows it to finish out the rest of the
    "undo" work properly.