MFV r277427:

Add more visibility via arcstats; specifically arc_state_t stats and
differentiate between "data" and "metadata".

Note that for now the FreeBSD implementation requires the observer
to poke at snapshot_bintime before reading the variables.

Illumos issue:

5445 Add more visibility via arcstats; specifically arc_state_t
     stats and differentiate between "data" and "metadata"