The `off` argument to `dirlookup` is optional.  It
was not being used in the `dirlookup` call in `create`,
so pass NULL instead and delete the unused local.

Signed-off-by: Dan Cross <cross@gajendra.net>

in main.c and don't make it disk specific; the icache is shared.  This
doesn't matter since we have only one disk, but conceptually cleaner
and maybe helpful to students for mount lab.

fs.html should perhaps be split in small homeworks as in previous
years in preparation for lectures and/or as demos during lecture.

several disks (e.g., in a mount lab).

they don't have to modify alarmtest.c, so we can use the
original version to test, to make it harder to cheat.

avoids handler freeing descriptors before driver sees that the request
has completed.

Use refcnt to pin blocks into the cache
Replace flags/B_VALID with a boolean field valid
Use info[id].status to signal completion of disk interrupt
Pass a read/write flag to virtio_disk_rw