Make smart_ulltoa return pointer to end (allows for code shink in callers)

Make smart_ulltoa return pointer to end (allows for code shink in callers)

function                                             old     new   delta
smart_ulltoa5                                        405     408      +3
smart_ulltoa4                                        273     276      +3
list_table                                          1113    1114      +1
scale                                                 36      34      -2
put_lu                                                55      53      -2
ulltoa6_and_space                                     19      14      -5
powertop_main                                       1470    1461      -9

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>