Skip calling CPU_ISSET for NOCPU as a short period of time we can have
td_oncpu is NOCPU for thread in TDS_RUNNING state.

Differential Revision:	https://reviews.freebsd.org/D1283
Reviewed by:	jhb
MFC after:	1 Month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275644 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f