This fix mostly matters after r206129 that made it possible for
st_blksize to be greater than 4K. For this reason, this change should
be MFC-ed before r206129.
Also, it seems that all FreeBSD uitlities that use db(3) hash databases
and create new databases in files, specify their own block size value
and thus do not depend on block size autotuning.
PR: bin/144446
Submitted by: Peter Jeremy <peterjeremy@acm.org>
MFC after: 5 days