vm_page_dump_index_to_pa(): Add braces to the expression involving + and &.

The precedence of the '&' operator is less than of '+'. Added braces
do change the order of evaluation into the natural one, in my opinion.
On the other hand, the value of the expression should not change since
all elements should have page-aligned values.

This fixes a gcc warning reported.

Reported by: adrian
Sponsored by: The FreeBSD Foundation
MFC after: 1 week