Correctly maintain the per-cpu field "curpmap" on amd64 just like we

Correctly maintain the per-cpu field "curpmap" on amd64 just like we
do on i386.  The consequences of not doing so on amd64 became apparent
with the introduction of the COUNT_IPIS and COUNT_XINVLTLB_HITS
options.  Specifically, single-threaded applications were generating
unnecessary IPIs to shoot-down the TLB on other processors.  However,
this is clearly nonsensical because a single-threaded application is
only running on the current processor.  The reason that this happens
is that pmap_activate() is unable to properly update the old pmap's
field "pm_active" without the correct "curpmap".  So, in effect, stale
bits in "pm_active" were leading pmap_protect(), pmap_remove(),
pmap_remove_pages(), etc. to flush the TLB contents on some arbitrary
processor that wasn't even running the same application.

Reviewed by:	kib
MFC after:	3 weeks