Illumos #3137 L2ARC compression

3137 L2ARC compression
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  illumos/illumos-gate@aad02571bc
  https://www.illumos.org/issues/3137
  http://wiki.illumos.org/display/illumos/L2ARC+Compression

Notes for Linux port:

A l2arc_nocompress module option was added to prevent the
compression of l2arc buffers regardless of how a dataset's
compression property is set.  This allows the legacy behavior
to be preserved.

Ported by: James H <james@kagisoft.co.uk>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1379

include/sys/dbuf.h

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
  */
 
 #ifndef	_SYS_DBUF_H
@@ -322,6 +323,9 @@ boolean_t dbuf_is_metadata(dmu_buf_impl_t *db);
 	(DBUF_IS_METADATA(_db) &&					\
 	((_db)->db_objset->os_secondary_cache == ZFS_CACHE_METADATA)))
 
+#define	DBUF_IS_L2COMPRESSIBLE(_db)					\
+	((_db)->db_objset->os_compress != ZIO_COMPRESS_OFF)
+
 #ifdef ZFS_DEBUG
 
 /*