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

sys/amd64/amd64/pmap.c

@@ -2111,7 +2111,7 @@ pmap_init(void)
 		 * Collect the page table pages that were replaced by a 2MB
 		 * page in create_pagetables().  They are zero filled.
 		 */
-		if (i << PDRSHIFT < KERNend &&
+		if ((vm_paddr_t)i << PDRSHIFT < KERNend &&
 		    pmap_insert_pt_page(kernel_pmap, mpte, false))
 			panic("pmap_init: pmap_insert_pt_page failed");
 	}