Change the logic around rounding up the needed memory for sysctls since

Change the logic around rounding up the needed memory for sysctls since
the network state can change between the two sysctl calls. Adding 10%
extra works for larger routing tables but can be too little on smaller
tables to hold even a single extra message. Instead of that add at least
1024 bytes or 10% (whichever is bigger) and round the size up to the next
page. With this there are no more sporadic errors in the bgpd integration
tests.
OK sthen@