sysctl: Index sysctl directories with rbtrees.

One of the most important jobs of sysctl is to export network stack
tunables.  Several of those tunables are per network device.  In
several instances people are running with 1000+ network devices in
there network stacks, which makes the simple per directory linked list
in sysctl a scaling bottleneck.   Replace O(N^2) sysctl insertion and
lookup times with O(NlogN) by using an rbtree to index the sysctl
directories.

Benchmark before:
    make-dummies 0 999 -> 0.32s
    rmmod dummy        -> 0.12s
    make-dummies 0 9999 -> 1m17s
    rmmod dummy         -> 17s

Benchmark after:
    make-dummies 0 999 -> 0.074s
    rmmod dummy        -> 0.070s
    make-dummies 0 9999 -> 3.4s
    rmmod dummy         -> 0.44s

Benchmark after (without dev_snmp6):
    make-dummies 0 9999 -> 0.75s
    rmmod dummy         -> 0.44s
    make-dummies 0 99999 -> 11s
    rmmod dummy          -> 4.3s

At 10,000 dummy devices the bottleneck becomes the time to add and
remove the files under /proc/sys/net/dev_snmp6.  I have commented
out the code that adds and removes files under /proc/sys/net/dev_snmp6
and taken measurments of creating and destroying 100,000 dummies to
verify the sysctl continues to scale.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>