s390/dasd: fix redundant /proc/dasd* entries removal

In case of an early failure in dasd_init, dasd_proc_init is never
called and /proc/dasd* files are never created. That can happen, for
example, if an incompatible or incorrect argument is provided to the
dasd_mod.dasd= kernel parameter.

However, the attempted removal of /proc/dasd* files causes 8 warnings
and backtraces in this case. 4 on the error path within dasd_init and
4 when the dasd module is unloaded. Notice the "removing permanent
/proc entry 'devices'" message that is caused by the dasd_proc_exit
function trying to remove /proc/devices instead of /proc/dasd/devices
since dasd_proc_root_entry is NULL and /proc/devices is indeed
permanent. Example:

------------[ cut here ]------------
removing permanent /proc entry 'devices'
WARNING: CPU: 6 PID: 557 at fs/proc/generic.c:701 remove_proc_entry+0x22e/0x240

CPU: 6 PID: 557 Comm: modprobe Not tainted 6.10.5-1-default #1
openSUSE Tumbleweed f6917bfd6e5a5c7a7e900e0e3b517786fb5c6301
Hardware name: QEMU 8561 QEMU (KVM/Linux)
Krnl PSW : 0704c00180000000 000003fffed0e9f2 (remove_proc_entry+0x232/0x240)
R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
Krnl GPRS: 000003ff00000027 000003ff00000023 0000000000000028 000002f200000000
000002f3f05bec20 0000037ffecfb7d0 000003ffffdabab0 000003ff7ee4ec72
000003ff7ee4ec72 0000000000000007 000002f280e22600 000002f280e22688
000003ffa252cfa0 0000000000010000 000003fffed0e9ee 0000037ffecfba38
Krnl Code: 000003fffed0e9e2: c020004e7017 larl %r2,000003ffff6dca10
000003fffed0e9e8: c0e5ffdfad24 brasl %r14,000003fffe904430
#000003fffed0e9ee: af000000 mc 0,0
>000003fffed0e9f2: a7f4ff4c brc 15,000003fffed0e88a
000003fffed0e9f6: 0707 bcr 0,%r7
000003fffed0e9f8: 0707 bcr 0,%r7
000003fffed0e9fa: 0707 bcr 0,%r7
000003fffed0e9fc: 0707 bcr 0,%r7
Call Trace:
[<000003fffed0e9f2>] remove_proc_entry+0x232/0x240
([<000003fffed0e9ee>] remove_proc_entry+0x22e/0x240)
[<000003ff7ef5a084>] dasd_proc_exit+0x34/0x60 [dasd_mod]
[<000003ff7ef560c2>] dasd_exit+0x22/0xc0 [dasd_mod]
[<000003ff7ee5a26e>] dasd_init+0x26e/0x280 [dasd_mod]
[<000003fffe8ac9d0>] do_one_initcall+0x40/0x220
[<000003fffe9bc758>] do_init_module+0x78/0x260
[<000003fffe9bf3a6>] __do_sys_init_module+0x216/0x250
[<000003ffff37ac9e>] __do_syscall+0x24e/0x2d0
[<000003ffff38cca8>] system_call+0x70/0x98
Last Breaking-Event-Address:
[<000003fffef7ea20>] __s390_indirect_jump_r14+0x0/0x10
---[ end trace 0000000000000000 ]---
------------[ cut here ]------------

While the cause is a user failure, the dasd module should handle the
situation more gracefully. One of the simplest solutions is to make
removal of the /proc/dasd* entries idempotent.

Signed-off-by: Miroslav Franc <[e-mail]>
[ sth: shortened if clause ]
Signed-off-by: Stefan Haberland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/s390/block/dasd_proc.c

···
 	remove_proc_entry("devices", dasd_proc_root_entry);
  out_nodevices:
 	remove_proc_entry("dasd", NULL);
+	dasd_proc_root_entry = NULL;
  out_nodasd:
 	return -ENOENT;
 }
···
 void
 dasd_proc_exit(void)
 {
+	if (!dasd_proc_root_entry)
+		return;
+
 	remove_proc_entry("devices", dasd_proc_root_entry);
 	remove_proc_entry("statistics", dasd_proc_root_entry);
 	remove_proc_entry("dasd", NULL);
+	dasd_proc_root_entry = NULL;
 }