MFV r258376: 3964 L2ARC should always compress metadata buffers

illumos/illumos-gate@e4be62a2b74a8f09bb669217a1a39eee069b13a1

MFC after:	10 days
Sponsored by:	HybridCluster [merge]

(cherry picked from commit b0eda9bba9e8f69f342a59f4beaf9e24398b4f64)
Disable .zfs dir totally