zstream: allow decompress to fix metadata for uncompressed records

If a record is uncompressed on-disk but the block pointer insists
otherwise, reading it will return EIO.  This commit adds an "off" type
to the "zstream decompress" command.  Using it will set the compression
field in a zfs stream to "off" without changing the record's data.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by:	Alan Somers <asomers@FreeBSD.org>
Sponsored by:	Axcient
Closes #13997

man/man8/zstream.8

@@ -20,7 +20,7 @@
 .\"
 .\" Copyright (c) 2020 by Delphix. All rights reserved.
 .\"
-.Dd March 25, 2022
+.Dd October 4, 2022
 .Dt ZSTREAM 8
 .Os
 .
@@ -96,6 +96,7 @@ Specify the object number and byte offset of each record that you wish to
 decompress.
 Optionally specify the compression type.
 Valid compression types include
+.Sy off ,
 .Sy gzip ,
 .Sy lz4 ,
 .Sy lzjb ,
@@ -108,6 +109,11 @@ Every record for that object beginning at that offset will be decompressed, if
 possible.
 It may not be possible, because the record may be corrupted in some but not
 all of the stream's snapshots.
+Specifying a compression type of
+.Sy off
+will change the stream's metadata accordingly, without attempting decompression.
+This can be useful if the record is already uncompressed but the metadata
+insists otherwise.
 The repaired stream will be written to standard output.
 .Bl -tag -width "-v"
 .It Fl v