MFC 216504:

MFC 216504:
Add back a bounds check on valid idle priorities that was lost in an
earlier commit.  While here, move the thread lock down in rtp_to_pri().
It is not needed for all of the priority value checks and the computation
of newpri.

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@216593 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f