ipc: always handle a new value of auto_msgmni

commit 1195d94e upstream.

proc_dointvec_minmax() returns zero if a new value has been set.  So we
don't need to check all charecters have been handled.

Below you can find two examples.  In the new value has not been handled
properly.

$ strace ./a.out
open("/proc/sys/kernel/auto_msgmni", O_WRONLY) = 3
write(3, "0\n\0", 3)                    = 2
close(3)                                = 0
exit_group(0)
$ cat /sys/kernel/debug/tracing/trace

$strace ./a.out
open("/proc/sys/kernel/auto_msgmni", O_WRONLY) = 3
write(3, "0\n", 2)                      = 2
close(3)                                = 0

$ cat /sys/kernel/debug/tracing/trace
a.out-697   [000] ....  3280.998235: unregister_ipcns_notifier <-proc_ipcauto_dointvec_minmax

Fixes: 9eefe520 ("ipc: do not use a negative value to re-enable msgmni automatic recomputin")
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Joe Perches <joe@perches.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

ipc/ipc_sysctl.c

@@ -123,7 +123,6 @@ static int proc_ipcauto_dointvec_minmax(struct ctl_table *table, int write,
 	void __user *buffer, size_t *lenp, loff_t *ppos)
 {
 	struct ctl_table ipc_table;
-	size_t lenp_bef = *lenp;
 	int oldval;
 	int rc;
 
@@ -133,7 +132,7 @@ static int proc_ipcauto_dointvec_minmax(struct ctl_table *table, int write,
 
 	rc = proc_dointvec_minmax(&ipc_table, write, buffer, lenp, ppos);
 
-	if (write && !rc && lenp_bef == *lenp) {
+	if (write && !rc) {
 		int newval = *((int *)(ipc_table.data));
 		/*
 		 * The file "auto_msgmni" has correctly been set.