hdf5_msb

Extract common subexpressions into `oent` and `nent` for brevity.

Assert index entries are in sorted order earlier in the loop over
old and new indices.

When looping over the remaining new index entries, just do
`entries_added++` to match the other loops.

In the final log entry in H5F_vfd_swmr_reader_end_of_tick(), mention
whether the call will exit with success or failure.