Fix some nits in 1G page support in the amd64 pmap.

Fix some nits in 1G page support in the amd64 pmap.

- Move assertions out of the main loop to avoid duplicate conditional
  expressions, and improve assertion messages.
- Fix va_next updates.  In some cases we were not doing the wraparound
  check before continuing the loop.
- Use the right va_next.  In pmap_advise() and pmap_copy() we would step
  through 1G pages 2M at a time.
- Copy 1G mappings in pmap_copy().

Reviewed by:	alc, kib
MFC with:	r365518
Sponsored by:	Juniper Networks, Inc., Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D26463