amd64 pmap: potential integer overflowing expression

Coverity has identified the line in this change as "Potential integer
overflowing expression" due to the variable i declared as an int
and used in an expression with vm_paddr_t, a 64bit variable.

This change has very little effect as when this line is execute
nkpt is small and phys_addr is a the beginning of physical memory.
But there is no explicit protection that the above is true.

Submitted by: bret_ketchum@dell.com
Reported by: Coverity
Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26141