kernel/sched: Fix spinlock validation glitch in SMP

In SMP, we are setting the _current pointer while holding the
scheduler spinlock locally, which means that when we try to release it
the validation layer (not the spinlock per se) will scream at us
because the thread that took the lock doesn't match the one releasing
it.

Special case this when validation is enabled.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>