Fix calculation of bits versus bytes for incremental bitmap

This is being separated out from WT-6136. While debugging that and looking at metadata I noticed that the number of bits needed to represent the file was incorrect causing us to over-allocate the bitmap needed.

This is a bug and needs to be fixed. It does not cause any correctness issues in the information reported in backup. It does means that we are wasting space in the information we store in metadata and we're doing more work walking the bitmap during backup to walk a lot of zero bits unnecessarily.