sched: Simplify sched_lend_user_prio_cond()

[olce]

Summary

If 'td_lend_user_pri' has the expected value, there is no need to check
the fields that sched_lend_user_prio() modifies, they either are already
good or soon will be ('td->td_lend_user_pri' has just been changed by
a concurrent update).