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/sys/zfs_context.h

@@ -492,7 +492,7 @@ extern taskq_t	*taskq_create(const char *, int, pri_t, int, int, uint_t);
 #define	taskq_create_proc(a, b, c, d, e, p, f) \
 	    (taskq_create(a, b, c, d, e, f))
 #define	taskq_create_sysdc(a, b, d, e, p, dc, f) \
-	    (taskq_create(a, b, maxclsyspri, d, e, f))
+	    ((void) sizeof (dc), taskq_create(a, b, maxclsyspri, d, e, f))
 extern taskqid_t taskq_dispatch(taskq_t *, task_func_t, void *, uint_t);
 extern taskqid_t taskq_dispatch_delay(taskq_t *, task_func_t, void *, uint_t,
     clock_t);