Fix a bug in synchronize RCU when the calling thread is bound to a CPU.

Set "td_pinned" to zero after "sched_unbind()" to prevent "td_pinned"
from temporarily becoming negative during "sched_bind()". This can
happen if "sched_bind()" uses "sched_pin()" and "sched_unpin()".

MFC after: 1 week
Sponsored by: Mellanox Technologies