HW5 add mutex for correct concurrent inserts
HW5 add mutex for correct concurrent inserts

The change in the `put` function was necessary because &table[i] never
changes after allocation, but table[i] does, so we need to either put
the lock/unlock calls around the `insert` call in `put` or do what I
did here, which I think is cleaner.

gw/xv6,gw/xv6,gw/xv6,gw/xv6,gw/xv6