metadump/restore: don't use errno after fwrite/fread failures

metadump/restore: don't use errno after fwrite/fread failures

fread/fwrite don't set errno, so printing out strerror(errno)
after a failure leads to incorrect and confusing messages:

# xfs_mdrestore pre_repair.meta pre_repair.img
xfs_mdrestore: error reading from file: Success

Don't return unset errno from write_index, either.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>