libgfs2: Allow init_dinode to accept a preallocated bh
    
    Previously init_dinode() always allocated the bh with bget() which meant
    less control over memory allocation higher up. This patch changes the
    signature of init_dinode to accept a bh pointer and only allocates a new
    bh if it is NULL. Also adds error checking to init_dinode()'s callers.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>