tty/sysrq: change the definition of sysrq_key_table's element to make it more readable

The definition of sysrq_key_table's elements, like sysrq_thaw_op and
sysrq_showallcpus_op are not consistent with sysrq_ftrace_dump_op,
Consistency makes code more readable.
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Junwen Wu <wudaemon@gmail.com>
Link: https://lore.kernel.org/r/20220418153703.97705-1-wudaemon@163.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/sysrq.c

@@ -274,6 +274,8 @@ static const struct sysrq_key_op sysrq_showallcpus_op = {
 	.action_msg	= "Show backtrace of all active CPUs",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
 };
+#else
+#define sysrq_showallcpus_op (*(const struct sysrq_key_op *)NULL)
 #endif
 
 static void sysrq_handle_showregs(int key)
@@ -405,6 +407,7 @@ static const struct sysrq_key_op sysrq_moom_op = {
 	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
 };
 
+#ifdef CONFIG_BLOCK
 static void sysrq_handle_thaw(int key)
 {
 	emergency_thaw_all();
@@ -415,6 +418,9 @@ static const struct sysrq_key_op sysrq_thaw_op = {
 	.action_msg	= "Emergency Thaw of all frozen filesystems",
 	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
 };
+#else
+#define sysrq_thaw_op (*(const struct sysrq_key_op *)NULL)
+#endif
 
 static void sysrq_handle_kill(int key)
 {
@@ -468,17 +474,9 @@ static const struct sysrq_key_op *sysrq_key_table[62] = {
 	NULL,				/* g */
 	NULL,				/* h - reserved for help */
 	&sysrq_kill_op,			/* i */
-#ifdef CONFIG_BLOCK
 	&sysrq_thaw_op,			/* j */
-#else
-	NULL,				/* j */
-#endif
 	&sysrq_SAK_op,			/* k */
-#ifdef CONFIG_SMP
 	&sysrq_showallcpus_op,		/* l */
-#else
-	NULL,				/* l */
-#endif
 	&sysrq_showmem_op,		/* m */
 	&sysrq_unrt_op,			/* n */
 	/* o: This will often be registered as 'Off' at init time */