zio_compress: introduce max size threshold

Now default compression is lz4, which can stop
compression process by itself on incompressible data.
If there are additional size checks -
we will only make our compressratio worse.

New usable compression thresholds are:
- less than BPE_PAYLOAD_SIZE (embedded_data feature);
- at least one saved sector.

Old 12.5% threshold is left to minimize affect
on existing user expectations of CPU utilization.

If data wasn't compressed - it will be saved as
ZIO_COMPRESS_OFF, so if we really need to recompress
data without ashift info and check anything -
we can just compress it with zero threshold.
So, we don't need a new feature flag here!

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #9416

man/man7/zfsprops.7

@@ -917,14 +917,24 @@ zeroes (the NUL byte).
 When a zero-filled block is detected, it is stored as
 a hole and not compressed using the indicated compression algorithm.
 .Pp
-Any block being compressed must be no larger than 7/8 of its original size
-after compression, otherwise the compression will not be considered worthwhile
-and the block saved uncompressed.
-Note that when the logical block is less than
-8 times the disk sector size this effectively reduces the necessary compression
-ratio; for example, 8 KiB blocks on disks with 4 KiB disk sectors must compress
-to 1/2
-or less of their original size.
+All blocks are allocated as a whole number of sectors
+.Pq chunks of 2^ Ns Sy ashift No bytes , e.g . Sy 512B No or Sy 4KB .
+Compression may result in a non-sector-aligned size, which will be rounded up
+to a whole number of sectors.
+If compression saves less than one whole sector,
+the block will be stored uncompressed.
+Therefore, blocks whose logical size is a small number of sectors will
+experience less compression
+(e.g. for
+.Sy recordsize Ns = Ns Sy 16K
+with
+.Sy 4K
+sectors, which have 4 sectors per block,
+compression needs to save at least 25% to actually save space on disk).
+.Pp
+There is
+.Sy 12.5%
+default compression threshold in addition to sector rounding.
 .It Xo
 .Sy context Ns = Ns Sy none Ns | Ns
 .Ar SELinux-User : Ns Ar SELinux-Role : Ns Ar SELinux-Type : Ns Ar Sensitivity-Level