Prevent arc_c collapse

Adjusting arc_c directly is racy because it can happen in the context
of multiple threads. It should always be >= 2 * maxblocksize. Set it
to a known valid value rather than adjusting it directly.

In addition refactor arc_shrink() to a simpler structure, protect against
underflow in the calculation of the new arc_c value.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reverts: 935434ef
Closes: #3904
Closes: #4161