Impact: make ftrace position computing more sane

First remove useless ->pos field. Then we needn't check seq_printf
in .show like other place.
Signed-off-by: NLiming Wang <liming.wang@windriver.com>
Reviewed-by: NBruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>