sbull: fix compiling errors

root cause commits in linux-stable:
commit 7988613b0e5b2638caf6cd493cc78e9595eba19c
Author: Kent Overstreet <kmo@daterainc.com>
Date:   Sat Nov 23 17:19:00 2013 -0800

    block: Convert bio_for_each_segment() to bvec_iter

    More prep work for immutable biovecs - with immutable bvecs drivers
    won't be able to use the biovec directly, they'll need to use helpers
    that take into account bio->bi_iter.bi_bvec_done.

    This updates callers for the new usage without changing the
    implementation yet.

commit 4550dd6c6b062fc5e5b647296d55da22616123c3
Author: Kent Overstreet <kmo@daterainc.com>
Date:   Wed Aug 7 14:26:21 2013 -0700

    block: Immutable bio vecs

    This adds a mechanism by which we can advance a bio by an arbitrary
    number of bytes without modifying the biovec: bio->bi_iter.bi_bvec_done
    indicates the number of bytes completed in the current bvec.

    Various driver code still needs to be updated to not refer to the bvec
    directly before we can use this for interesting things, like efficient
    bio splitting.

commit 4f024f3797c43cb4b73cd2c50cec728842d0e49e
Author: Kent Overstreet <kmo@daterainc.com>
Date:   Fri Oct 11 15:44:27 2013 -0700

    block: Abstract out bvec iterator

    Immutable biovecs are going to require an explicit iterator. To
    implement immutable bvecs, a later patch is going to add a bi_bvec_done
    member to this struct; for now, this patch effectively just renames
    things.