rbd.cc: tolerate lack of NUL-termination on block_name_prefix

Fixes: #7577
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit fd76fec589be13a4a6362ef388929d3e3d1d21f6)
Fix byte-order dependency in calculation of initial challenge

Fixes: #7977
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4dc62669ecd679bc4d0ef2b996b2f0b45b8b4dc7)