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)
expose zfs_flags as debug.zfs_flags r/w tunable and sysctl

This knob is purposefully hidden under debug.

MFC after:	5 days
Sponsored by:	HybridCluster

(cherry picked from commit aab537bd024eb79d37f1fc3862dd3df05b158933)
fix debug.zfs_flags sysctl description in r258638

Pointyhat to:	avg
MFC after:	3 days

(cherry picked from commit 1729bafbfeb18dd6aea9afffe11b5f181396561f)