Increase default volblocksize from 8KB to 16KB

Many things has changed since previous default was set many years ago.
Nowadays 8KB does not allow adequate compression or even decent space
efficiency on many of pools due to 4KB disk physical block rounding,
especially on RAIDZ and DRAID.  It effectively limits write throughput
to only 2-3GB/s (250-350K blocks/s) due to sync thread, allocation,
vdev queue and other block rate bottlenecks.  It keeps L2ARC expensive
despite many optimizations and dedup just unrealistic.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #12406

man/man7/zfsprops.7

@@ -527,7 +527,7 @@ cannot be changed once the volume has been written, so it should be set at
 volume creation time.
 The default
 .Sy blocksize
-for volumes is 8 Kbytes.
+for volumes is 16 Kbytes.
 Any power of 2 from 512 bytes to 128 Kbytes is valid.
 .Pp
 This property can also be referred to by its shortened column name,