Release topology restrictions on special/dedup

Special vdevs were originally designed as a small blocks storage
for dRAID, for which role RAIDZ/dRAID topologies are not good.
But it is more often used as SSD storage for metadata and hot
data of HDD pools.  In these use cases narrow RAIDZ of SSDs might
be fine, so we should not introduce unnecessary restrictions,
and ZFS internally does not care.

Similar applies to dedup vdevs.  Original DDT used 4KB blocks,
for which anything but mirror was a terrible storage.  But new
FDT implementation uses 32KB blocks by default, which are much
less demanding even including compression, and which could be
increased even higher now, if needed.

Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.

Author: amotin

cmd/zpool/zpool_vdev.c

@@ -1581,13 +1581,12 @@ construct_spec(nvlist_t *props, int argc, char **argv)
 				is_dedup = is_spare = B_FALSE;
 			}
 
-			if (is_log || is_special || is_dedup) {
+			if (is_log) {
 				if (strcmp(type, VDEV_TYPE_MIRROR) != 0) {
 					(void) fprintf(stderr,
 					    gettext("invalid vdev "
-					    "specification: unsupported '%s' "
-					    "device: %s\n"), is_log ? "log" :
-					    "special", type);
+					    "specification: unsupported 'log' "
+					    "device: %s\n"), type);
 					goto spec_out;
 				}
 				nlogs++;