fs: simple getdtablesize implementation

man page says: "returns the maximum number of files a process can have open".
We're not really doing any limit tuning, so this is equivalent to FDMAX in our
case.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>