printk: fix build warning when CONFIG_PRINTK=n

build warning when CONFIG_PRINTK=n

	kernel/printk/printk.c:175:5: warning: no previous prototype for
	 'devkmsg_sysctl_set_loglvl' [-Wmissing-prototypes]

devkmsg_sysctl_set_loglvl() is only used in sysctl.c when
CONFIG_PRINTK=y, but it participates in the build when CONFIG_PRINTK=n.
So add compile dependency CONFIG_PRINTK=y && CONFIG_SYSCTL=y to fix the
build warning.

Link: https://lkml.kernel.org/r/20211129211943.640266-5-mcgrof@kernel.org
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: Antti Palosaari <crope@iki.fi>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Iurii Zaikin <yzaikin@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Lukas Middendorf <kernel@tuxforce.de>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Stephen Kitt <steve@sk2.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: nixiaoming

include/linux/printk.h

@@ -183,10 +183,6 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
 extern int printk_delay_msec;
 extern int dmesg_restrict;
 
-extern int
-devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write, void *buf,
-			  size_t *lenp, loff_t *ppos);
-
 extern void wake_up_klogd(void);
 
 char *log_buf_addr_get(void);

kernel/printk/internal.h

@@ -6,6 +6,8 @@
 
 #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
 void __init printk_sysctl_init(void);
+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+			      void *buffer, size_t *lenp, loff_t *ppos);
 #else
 #define printk_sysctl_init() do { } while (0)
 #endif

kernel/printk/printk.c

@@ -171,7 +171,7 @@ static int __init control_devkmsg(char *str)
 __setup("printk.devkmsg=", control_devkmsg);
 
 char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
-
+#if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
 int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 			      void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -210,6 +210,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 
 	return 0;
 }
+#endif /* CONFIG_PRINTK && CONFIG_SYSCTL */
 
 /* Number of registered extended console drivers. */
 static int nr_ext_console_drivers;