kernel: mutex: delay setting lock_count = 0.

It is necessary to delay setting lock_count = 0 because an unlocking thread
maybe swapped out when it calls adjust_owner_prio(). If the thread that starts
running sees lock_count = 0 it will successfully acquire the mutex even though
it is not fully unlocked yet.

Fixes #11798.

Signed-off-by: Nicolás Bértolo <nicolasbertolo@gmail.com>