cd9660_open should check for padding

[tsoome]

Summary

I have "buggy" iso, where there is no /boot/loader.conf file, but loader
does not fail with file not found, but instead, it does open
/boot/defaults/loader.conf.

While investigating I found:

1. the directory extent for /boot is 4096 bytes
2. the last file in directory, /boot/zfsloader, ends at extent offset 2048.
3. the next entry has length 0 (and in fact, all the directory entry is zero).

So we have hit the corner case with second half of the directory extent is
actually padding.

Test Plan

With patch applied, the missing /boot/loader.conf is correctly reported
as not found.

[allanjude]

Approved for commit

Remember to include 'reviewed by: jhb' in the commit message