vmapbuf: don't smuggle address or length in buf

vmapbuf: don't smuggle address or length in buf

Instead, add arguments to vmapbuf.  Since this argument is
always a pointer use a type of void * and cast to vm_offset_t in
vmapbuf.  (In CheriBSD we've altered vm_fault_quick_hold_pages to
take a pointer and check its bounds.)

In no other situtation does b_data contain a user pointer and vmapbuf
replaces b_data with the actual mapping.

Suggested by:	jhb
Reviewed by:	imp, jhb
Obtained from:	CheriBSD
MFC after:	1 week
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D26784