zstream: allow decompress to fix metadata for uncompressed records

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