xfs_scrub: fix read verify disk error handling strategy

xfs_scrub: fix read verify disk error handling strategy

The error handling strategy for media errors is totally bogus.  First of
all, short reads are entirely unhandled -- when we encounter a short
read, we know the disk was able to feed us the beginning of what we
asked for, so we need to single-step through the remainder to try to
capture the exact error that we hit.

Second, an actual IO error causes the entire region to be marked bad
even though it could be just a few MB of a multi-gigabyte extent that's
bad.  Therefore, single-step each block in the IO request until we stop
getting IO errors to find out if all the blocks are bad or if it's just
that extent.

Third, fix the fact that the loop updates its own counter variables with
the length fed to read(), which doesn't necessarily have anything to do
with the amount of data that the read actually produced.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
[sandeen: change "io_error" to "read_error"]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>