FreeBSD: Fix regression from kmem_scnprintf() in libzfs

kmem_scnprintf() is only available in libzpool. Recent buildbot issues
with showing FreeBSD results kept us from seeing this before
97143b9 was merged.

The code has been changed to sanitize the output from `kmem_scnprintf()`.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14111

lib/libzfs/os/freebsd/libzfs_compat.c

@@ -202,8 +202,10 @@ libzfs_error_init(int error)
 	size_t msglen = sizeof (errbuf);
 
 	if (modfind("zfs") < 0) {
-		size_t len = kmem_scnprintf(msg, msglen, dgettext(TEXT_DOMAIN,
+		size_t len = snprintf(msg, msglen, dgettext(TEXT_DOMAIN,
 		    "Failed to load %s module: "), ZFS_KMOD);
+		if (len >= msglen)
+			len = msglen - 1;
 		msg += len;
 		msglen -= len;
 	}