MFV r251474:

 * Illumos zfs issue #3137 L2ARC compression

Whether or not to compress buffers entering the L2ARC is
controlled by "compression" setting on the dataset, when
compression is not "off", L2ARC compression is enabled.

The compress method is always LZ4 for L2ARC when enabled
because it works best for the scenario.

MFC after:	2 weeks