libspl: consult ZFS_HOSTID on FreeBSD as well

get_system_hostid() returns gethostid() on FreeBSD, so ZFS_HOSTID has
no effect there, while on Linux it overrides the SPL hostid.  zloop.sh
exports it for the iterations which need a hostid, and the ztest(1)
ENVIRONMENT VARIABLES section describes it without restricting it to
one platform, so both should read it.

Check ZFS_HOSTID first and fall back to gethostid(), parsing and
masking the value exactly as the Linux implementation does.  A value
which parses as zero is ignored there, so it is ignored here too.

Drop the "On Linux" qualifier from the -M description in ztest(1),
which this change makes untrue.

Suggested-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Michael Heller <[email protected]>
Closes #18918
Closes #18964

lib/libspl/os/freebsd/gethostid.c

@@ -23,5 +23,19 @@
 unsigned long
 get_system_hostid(void)
 {
+	char *env;
+
+	/*
+	 * Allow the hostid to be subverted for testing.  A value which
+	 * parses as zero is ignored, as it is on Linux, so that the
+	 * system hostid is used instead.
+	 */
+	env = getenv("ZFS_HOSTID");
+	if (env != NULL) {
+		unsigned long hostid = strtoull(env, NULL, 0);
+		if (hostid != 0)
+			return (hostid & HOSTID_MASK);
+	}
+
 	return (gethostid());
 }

man/man1/ztest.1

@@ -157,7 +157,7 @@ Multi-host; create the pool with the
 .Sy multihost
 property enabled, so that subsequent imports run the MMP activity check.
 Requires a non-zero hostid.
-On Linux one may be supplied through
+One may be supplied through
 .Ev ZFS_HOSTID .
 .It Fl E , -use-existing-pool
 Use existing pool (use existing pool instead of creating new one).