Remove dedupditto functionality

If dedup is in use, the `dedupditto` property can be set, causing ZFS to
keep an extra copy of data that is referenced many times (>100x).  The
idea was that this data is more important than other data and thus we
want to be really sure that it is not lost if the disk experiences a
small amount of random corruption.

ZFS (and system administrators) rely on the pool-level redundancy to
protect their data (e.g. mirroring or RAIDZ).  Since the user/sysadmin
doesn't have control over what data will be offered extra redundancy by
dedupditto, this extra redundancy is not very useful.  The bulk of the
data is still vulnerable to loss based on the pool-level redundancy.
For example, if particle strikes corrupt 0.1% of blocks, you will either
be saved by mirror/raidz, or you will be sad.  This is true even if
dedupditto saved another 0.01% of blocks from being corrupted.

Therefore, the dedupditto functionality is rarely enabled (i.e. the
property is rarely set), and it fulfills its promise of increased
redundancy even more rarely.

Additionally, this feature does not work as advertised (on existing
releases), because scrub/resilver did not repair the extra (dedupditto)
copy (see https://github.com/zfsonlinux/zfs/pull/8270).

In summary, this seldom-used feature doesn't work, and even if it did it
wouldn't provide useful data protection.  It has a non-trivial
maintenance burden (again see https://github.com/zfsonlinux/zfs/pull/8270).

We should remove the dedupditto functionality.  For backwards
compatibility with the existing CLI, "zpool set dedupditto" will still
"succeed" (exit code zero), but won't have any effect.  For backwards
compatibility with existing pools that had dedupditto enabled at some
point, the code will still be able to understand dedupditto blocks and
free them when appropriate.  However, ZFS won't write any new dedupditto
blocks.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Issue #8270 
Closes #8310

include/sys/ddt.h

@@ -103,6 +103,10 @@ typedef struct ddt_phys {
 	uint64_t	ddp_phys_birth;
 } ddt_phys_t;
 
+/*
+ * Note, we no longer generate new DDT_PHYS_DITTO-type blocks.  However,
+ * we maintain the ability to free existing dedup-ditto blocks.
+ */
 enum ddt_phys_type {
 	DDT_PHYS_DITTO = 0,
 	DDT_PHYS_SINGLE = 1,
@@ -216,10 +220,6 @@ extern void ddt_get_dedup_stats(spa_t *spa, ddt_stat_t *dds_total);
 extern uint64_t ddt_get_dedup_dspace(spa_t *spa);
 extern uint64_t ddt_get_pool_dedup_ratio(spa_t *spa);
 
-extern int ddt_ditto_copies_needed(ddt_t *ddt, ddt_entry_t *dde,
-    ddt_phys_t *ddp_willref);
-extern int ddt_ditto_copies_present(ddt_entry_t *dde);
-
 extern size_t ddt_compress(void *src, uchar_t *dst, size_t s_len, size_t d_len);
 extern void ddt_decompress(uchar_t *src, void *dst, size_t s_len, size_t d_len);
 

include/sys/spa_impl.h

@@ -350,7 +350,6 @@ struct spa {
 	ddt_t		*spa_ddt[ZIO_CHECKSUM_FUNCTIONS]; /* in-core DDTs */
 	uint64_t	spa_ddt_stat_object;	/* DDT statistics */
 	uint64_t	spa_dedup_dspace;	/* Cache get_dedup_dspace() */
-	uint64_t	spa_dedup_ditto;	/* dedup ditto threshold */
 	uint64_t	spa_dedup_checksum;	/* default dedup checksum */
 	uint64_t	spa_dspace;		/* dspace in normal class */
 	kmutex_t	spa_vdev_top_lock;	/* dueling offline/remove */

include/sys/zio.h

@@ -102,7 +102,6 @@ enum zio_checksum {
 #define	ZIO_CHECKSUM_VERIFY	(1 << 8)
 
 #define	ZIO_DEDUPCHECKSUM	ZIO_CHECKSUM_SHA256
-#define	ZIO_DEDUPDITTO_MIN	100
 
 /* supported encryption algorithms */
 enum zio_encrypt {