module/*.ko: prune .data, global .rodata

Evaluated every variable that lives in .data (and globals in .rodata)
in the kernel modules, and constified/eliminated/localised them
appropriately. This means that all read-only data is now actually
read-only data, and, if possible, at file scope. A lot of previously-
global-symbols became inlinable (and inlined!) constants. Probably
not in a big Wowee Performance Moment, but hey.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12899

include/os/linux/spl/sys/sysmacros.h

@@ -113,7 +113,6 @@
 #endif
 
 /* Missing globals */
-extern char spl_gitrev[64];
 extern unsigned long spl_hostid;
 
 /* Missing misc functions */

include/os/linux/spl/sys/taskq.h

@@ -160,7 +160,8 @@ extern taskq_t *taskq_of_curthread(void);
 #define	taskq_create_proc(name, nthreads, pri, min, max, proc, flags) \
     taskq_create(name, nthreads, pri, min, max, flags)
 #define	taskq_create_sysdc(name, nthreads, min, max, proc, dc, flags) \
-    taskq_create(name, nthreads, maxclsyspri, min, max, flags)
+	((void) sizeof (dc), \
+	    taskq_create(name, nthreads, maxclsyspri, min, max, flags))
 
 int spl_taskq_init(void);
 void spl_taskq_fini(void);