There is no reason why we need to pin the underlying thread to its current
processor in pmap_invalidate_{all,page,range}(). These functions are using
an instruction that broadcasts the TLB invalidation to every processor, so
even if a thread migrates in the middle of one of these functions every
processor will still perform the required TLB invalidations.

Reviewed by: andrew, markj
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D22502