libkvm: fix kvm_walk_pages
Correct bitmap operations in _kvm_bitmap_next.
Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19183
(cherry picked from commit 08055452cbf24a3cea48cb4f665bab78d89b7a39)