badblocks: Only report errors when reading/writing one block at a time

badblocks: Only report errors when reading/writing one block at a time

With Direct I/O, the kernel can report 0 bytes read even though the
first block has no errors.  So there are any errors, we need try to
read/write blocks one at a time and to get an accurate report.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>