fsck.gfs2: remove duplicate designation during undo
    
    This patch fixes a problem whereby fsck.gfs2's pass1 would perform this
    sequence of events:
    1. Metadata block X is identified as being referenced from dinode D1.
    2. Metadata block X is identified as being referenced from another dinode, D2,
       which makes it a duplicate reference, but so far, no serious errors were
       found for that dinode.
    3. Dinode D2 is found later to be irreparably damaged, and needs to be removed.
    When D2 is deleted, the duplicate reference from D2 is removed and block X is
    not freed because D1 still references it. However, it's still marked as a
    duplicate and requires processing in pass1b.
    
    Later, pass1b resolves the duplicate and determine's there is really only one
    reference remaining, so it makes the correct decision. However, it should not
    be necessary. The "undo" functions should remove the duplicate reference if
    (and only if) the only reference was from D2. Note, though, that if the
    corruption is found later in the cycle (after "undo" is possible) the duplicate
    reference MUST remain and be resolved by pass1b.