dtrace: add a knob to control maximum size of principal buffers

We had a hardcoded limit of 1/128-th of physical memory that was further
subdivided between all CPUs as principal buffers are allocated on the
per-CPU basis. Actually, the buffers could use up 1/64-th of the
memmory because with the default switch policy there are two buffers per
CPU.

This commit allows to change that limit.

Note that the discussed limit is per dtrace command invocation.
The idea is to limit the size of a single malloc(9) call, not the total
memory size used by DTrace buffers.

(cherry picked from commit 7fdf0e883567d96d27d929c49a74d5fdb84d550b)