kfence: default to dynamic branch instead of static keys mode

We have observed that on very large machines with newer CPUs, the static
key/branch switching delay is on the order of milliseconds.  This is due
to the required broadcast IPIs, which simply does not scale well to
hundreds of CPUs (cores).  If done too frequently, this can adversely
affect tail latencies of various workloads.

One workaround is to increase the sample interval to several seconds,
while decreasing sampled allocation coverage, but the problem still
exists and could still increase tail latencies.

As already noted in the Kconfig help text, there are trade-offs: at
lower sample intervals the dynamic branch results in better performance;
however, at very large sample intervals, the static keys mode can result
in better performance -- careful benchmarking is recommended.

Our initial benchmarking showed that with large enough sample intervals
and workloads stressing the allocator, the static keys mode was slightly
better.  Evaluating ...