Check args using the same expression as in fdesc and kernfs. The check
was actually already correct, modulo overflow. It could be tightened
up to either allow huge (aligned) offsets, treating them as EOF, or
disallow all offsets beyond EOF.
Didn't fix invalid address calculation &foo[i] where i may be out of
bounds.
Didn't fix shooting of foot using a private unportable dirent struct.
a336cb95ff