Fix a silly bug in dev/root.c

We can now make up to 256 files in bin and lib. It's quite fast --
much faster than our current root file system, as measured:
cat /bin/ash > /9/bin/ash
cmp /bin/ash /bin/ash: 15 seconds
cmp /9/bin/ash /9/bin/ash: too fast to tell

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>