netfilter: conntrack: allow increasing bucket size via sysctl too

No need to restrict this to module parameter.

We export a copy of the real hash size -- when user alters the value we
allocate the new table, copy entries etc before we update the real size
to the requested one.

This is also needed because the real size is used by concurrent readers
and cannot be changed without synchronizing the conntrack generation
seqcnt.

We only allow changing this value from the initial net namespace.

Tested using http-client-benchmark vs. httpterm with concurrent

while true;do
 echo $RANDOM > /proc/sys/net/netfilter/nf_conntrack_buckets
done

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Documentation/networking/nf_conntrack-sysctl.txt

@@ -7,12 +7,13 @@ nf_conntrack_acct - BOOLEAN
 	Enable connection tracking flow accounting. 64-bit byte and packet
 	counters per flow are added.
 
-nf_conntrack_buckets - INTEGER (read-only)
+nf_conntrack_buckets - INTEGER
 	Size of hash table. If not specified as parameter during module
 	loading, the default size is calculated by dividing total memory
 	by 16384 to determine the number of buckets but the hash table will
 	never have fewer than 32 and limited to 16384 buckets. For systems
 	with more than 4GB of memory it will be 65536 buckets.
+	This sysctl is only writeable in the initial net namespace.
 
 nf_conntrack_checksum - BOOLEAN
 	0 - disabled