OpenZFS 6322 - ZFS indirect block predictive prefetch

For quite some time I was thinking about possibility to prefetch
ZFS indirection tables while doing sequential reads or writes.
Recent changes in predictive prefetcher made that much easier to
do. My tests on zvol with 16KB block size on 5x striped and 2x
mirrored pool of 10 disks show almost double throughput on sequential
read, and almost tripple on sequential rewrite. While for read alike
effect can be received from increasing maximal prefetch distance
(though at higher memory cost), for rewrite there is no other
solution so far.

Authored by: Alexander Motin <mav@freebsd.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6322
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/cb92f413
Closes #5040

Porting notes:
- Change from upstream in module/zfs/dbuf.c in 'int dbuf_read' due
  to commit 5f6d0b6 'Handle block pointers with a corrupt logical size'

- Difference from upstream in module/zfs/dmu_zfetch.c,
  uint32_t zfetch_max_idistance -> unsigned int zfetch_max_idistance

- Variables have been initialized at the beginning of the function
 (void dmu_zfetch) to resemble the order of occurrence and account
 for C99, C11 mode errors.

include/sys/dmu_zfetch.h

@@ -43,6 +43,13 @@ struct dnode;				/* so we can reference dnode */
 typedef struct zstream {
 	uint64_t	zs_blkid;	/* expect next access at this blkid */
 	uint64_t	zs_pf_blkid;	/* next block to prefetch */
+
+	/*
+	 * We will next prefetch the L1 indirect block of this level-0
+	 * block id.
+	 */
+	uint64_t	zs_ipf_blkid;
+
 	kmutex_t	zs_lock;	/* protects stream */
 	hrtime_t	zs_atime;	/* time last prefetch issued */
 	list_node_t	zs_node;	/* link for zf_stream */
@@ -59,7 +66,7 @@ void		zfetch_fini(void);
 
 void		dmu_zfetch_init(zfetch_t *, struct dnode *);
 void		dmu_zfetch_fini(zfetch_t *);
-void		dmu_zfetch(zfetch_t *, uint64_t, uint64_t);
+void		dmu_zfetch(zfetch_t *, uint64_t, uint64_t, boolean_t);
 
 
 #ifdef	__cplusplus

include/sys/dnode.h

@@ -349,6 +349,15 @@ int dnode_next_offset(dnode_t *dn, int flags, uint64_t *off,
 void dnode_evict_dbufs(dnode_t *dn);
 void dnode_evict_bonus(dnode_t *dn);
 
+#define	DNODE_IS_CACHEABLE(_dn)						\
+	((_dn)->dn_objset->os_primary_cache == ZFS_CACHE_ALL ||		\
+	(DMU_OT_IS_METADATA((_dn)->dn_type) &&				\
+	(_dn)->dn_objset->os_primary_cache == ZFS_CACHE_METADATA))
+
+#define	DNODE_META_IS_CACHEABLE(_dn)					\
+	((_dn)->dn_objset->os_primary_cache == ZFS_CACHE_ALL ||		\
+	(_dn)->dn_objset->os_primary_cache == ZFS_CACHE_METADATA)
+
 #ifdef ZFS_DEBUG
 
 /*