Add a VA_IS_CLEANMAP() macro.

Add a VA_IS_CLEANMAP() macro.

This macro returns true if a provided virtual address is contained
in the kernel's clean submap.

In CHERI kernels, the buffer cache and transient I/O map are allocated
as separate regions.  Abstracting this check reduces the diff relative
to FreeBSD.  It is perhaps slightly more readable as well.

Reviewed by:	kib
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D28710

(cherry picked from commit 67932460c7b6893a637a47d98d5f29d63e92c727)