vmspace: Convert to refcount(9)

vmspace: Convert to refcount(9)

This is mostly mechanical except for vmspace_exit().  There, use the new
refcount_release_if_last() to avoid switching to vmspace0 unless other
processes are sharing the vmspace.  In that case, upon switching to
vmspace0 we can unconditionally release the reference.

Remove the volatile qualifier from vm_refcnt now that accesses are
protected using refcount(9) KPIs.

Reviewed by:	alc, kib, mmel
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27057