vm_map: overlap system map mutex and user man sx

This saves 616-584 = 32 bytes per struct vmspace on amd64, which allows
to pack 7 vmspaces per page vs. 6 for non-overlapping layout.

I used anonymous union member feature to avoid too much churn.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47934