Rename kern_mmap_req to kern_mmap

Replace all uses of kern_mmap with kern_mmap_req move the old kern_mmap.
Reand rename kern_mmap_req to kern_mmap .

The helper saved some code churn initially, but having multiple
interfaces is sub-optimal.

Obtained from: CheriBSD
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D28292