arp/ndp default hash size

[Alexander V. Chernikov]

Hello list!

Currently size of arp/ndp hash is the following:
#define    LLTBL_HASHTBL_SIZE      32      /* default 32 ? */

This may be OK for end hosts, but this is definitely not enough for 
router howadays. Especially given that IPv6 hosts generate 2 ndp records.

Output from 2 random v4 / v6 routers from my $job:
2:23 [0] m at matisse arp -an | wc -l
     1494
2:24 [0] m at singapore ndp -an | wc -l
     3999

Given that LIST_HEAD is just a pointer, and we currently have single 
global (actually per-VNET) instance for every l3 proto, bumping hash to, 
say, at least 4096 should do no harm.