hdf5_msb

Reduce code duplication by using vfd_swmr_pageno_to_mdf_idx_entry() to look up
shadow pages.  Reduce casts by choosing correct format strings and compatible
variable types.

Poison writes to addr by making it const.  Don't increase addr in the read(2)
loop because it's never used afterward.

Delete some more dead code.

Rename read_ptr as p and declare it much closer to its use.  Change its type to
`char *` so that no casts are necessary to increase it.