linuxkpi: Add <linux/ascii85.h>

This is used by the i915 DRM driver for some time to log more details
about a GPU error, but the code was commented out.

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56282

sys/compat/linuxkpi/common/include/linux/ascii85.h

@@ -0,0 +1,46 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2026 The FreeBSD Foundation
+ */
+
+#ifndef	_LINUXKPI_LINUX_ASCII85_H_
+#define	_LINUXKPI_LINUX_ASCII85_H_
+
+#include <sys/param.h>
+
+#define	ASCII85_BUFSZ 6
+
+static inline long
+ascii85_encode_len(long in_len)
+{
+	long out_len;
+
+	out_len = howmany(in_len, 4);
+
+	return (out_len);
+}
+
+static inline const char *
+ascii85_encode(uint32_t in, char *out)
+{
+	int i;
+
+	if (in == 0) {
+		out[0] = 'z';
+		out[1] = '\0';
+		return (out);
+	}
+
+	for (i = ASCII85_BUFSZ - 2; i >= 0; i--) {
+		out[i] = in % 85;
+		out[i] += 33;
+
+		in /= 85;
+	}
+	out[ASCII85_BUFSZ - 1] = '\0';
+
+	return (out);
+}
+
+#endif /* _LINUXKPI_LINUX_ASCII85_H_ */