Correct calculation of extent descriptor array size

dasdseq.c:178:45: warning: sizeof on array function parameter will return size
      of 'DSXTENT *' (aka 'struct DSXTENT *') instead of 'DSXTENT []'
      [-Wsizeof-array-argument]
            data_dump((void *)extent, sizeof(extent) * MAX_EXTENTS);
                                            ^
dasdseq.c:100:33: note: declared here
                DSXTENT         extent[], 
                                ^