net: atm: bring back zatm uAPI

Jiri reports that linux-atm does not build without this header.
Bring it back. It's completely dead code but we can't break
the build for user space :(
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Fixes: 052e1f01 ("net: atm: remove support for ZeitNet ZN122x ATM devices")
Link: https://lore.kernel.org/all/8576aef3-37e4-8bae-bab5-08f82a78efd3@kernel.org/
Link: https://lore.kernel.org/r/20220810164547.484378-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/atm_zatm.h

+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/* atm_zatm.h - Driver-specific declarations of the ZATM driver (for use by
+		driver-specific utilities) */
+
+/* Written 1995-1999 by Werner Almesberger, EPFL LRC/ICA */
+
+
+#ifndef LINUX_ATM_ZATM_H
+#define LINUX_ATM_ZATM_H
+
+/*
+ * Note: non-kernel programs including this file must also include
+ * sys/types.h for struct timeval
+ */
+
+#include <linux/atmapi.h>
+#include <linux/atmioc.h>
+
+#define ZATM_GETPOOL	_IOW('a',ATMIOC_SARPRV+1,struct atmif_sioc)
+						/* get pool statistics */
+#define ZATM_GETPOOLZ	_IOW('a',ATMIOC_SARPRV+2,struct atmif_sioc)
+						/* get statistics and zero */
+#define ZATM_SETPOOL	_IOW('a',ATMIOC_SARPRV+3,struct atmif_sioc)
+						/* set pool parameters */
+
+struct zatm_pool_info {
+	int ref_count;			/* free buffer pool usage counters */
+	int low_water,high_water;	/* refill parameters */
+	int rqa_count,rqu_count;	/* queue condition counters */
+	int offset,next_off;		/* alignment optimizations: offset */
+	int next_cnt,next_thres;	/* repetition counter and threshold */
+};
+
+struct zatm_pool_req {
+	int pool_num;			/* pool number */
+	struct zatm_pool_info info;	/* actual information */
+};
+
+#define ZATM_OAM_POOL		0	/* free buffer pool for OAM cells */
+#define ZATM_AAL0_POOL		1	/* free buffer pool for AAL0 cells */
+#define ZATM_AAL5_POOL_BASE	2	/* first AAL5 free buffer pool */
+#define ZATM_LAST_POOL	ZATM_AAL5_POOL_BASE+10 /* max. 64 kB */
+
+#define ZATM_TIMER_HISTORY_SIZE	16	/* number of timer adjustments to
+					   record; must be 2^n */
+
+#endif