MFV r337212:
9465 ARC check for 'anon_size > arc_c/2' can stall the system

illumos/illumos-gate@abe1fd01ce5a83718c5a840daeab4abdaec1c104

Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Don Brady <don.brady@delphix.com>