sched: Fix out of scope variable access in sched_slice()

Access to local variable lw is aliased by usage of pointer load.
Access to pointer load in calc_delta_mine() happens when lw is
already out of scope.

[ Reported by static code analysis. ]

Signed-off-by: Christian Engelmayer <christian.engelmayer@frequentis.com>
LKML-Reference: <20090616103512.0c846e51@frequentis.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>