Added test for being able to read various variantss of zstd

As detailed in openzfs#12022 and openzfs#12008, it turns out the current zstd implementation
is quite nonportable, and results in various configurations of ondisk header
that only each platform can read.

So I've added a test which contains a dataset with a file written by
Linux/x86_64, and one written by current FBSD/ppc64.

(I'll add one for Linux/sparc64 once the unmodified git checkout finishes
building. Linux/ppc64 behaves identically to Linux/sparc64.)

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

tests/runfiles/sanity.run

@@ -413,7 +413,7 @@ user =
 tags = ['functional', 'cli_user', 'zpool_list']
 
 [tests/functional/compression]
-tests = ['compress_003_pos']
+tests = ['compress_003_pos','compress_zstd_bswap']
 tags = ['functional', 'compression']
 
 [tests/functional/exec]

tests/zfs-tests/tests/functional/compression/compress_zstd_bswap.ksh

@@ -0,0 +1,55 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2007, Sun Microsystems Inc. All rights reserved.
+# Copyright (c) 2021, Rich Ercolani.
+# Use is subject to license terms.
+#
+
+. $STF_SUITE/include/properties.shlib
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# TODO
+
+typeset TESTPOOL_ZSTD_FILE=$STF_SUITE/tests/functional/compression/testpool_zstd.tar.gz
+verify_runnable "both"
+
+function cleanup
+{
+	pool_destroy testpool_zstd
+	rm -f $TEST_BASE_DIR/testpool_zstd
+
+}
+
+log_assert "Trying to read data from variously mangled zstd datasets"
+log_onexit cleanup
+
+tar --directory $TEST_BASE_DIR -xzSf $TESTPOOL_ZSTD_FILE
+zpool import -d $TEST_BASE_DIR testpool_zstd
+log_must cp /testpool_zstd/x86_64/zstd /dev/null
+#log_must cp /testpool_zstd/sparc64/zstd /dev/null
+log_must cp /testpool_zstd/ppc64_fbsd/zstd /dev/null
+
+log_pass "Reading from mangled zstd datasets works as expected."