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/zfs_comutil.h

@@ -43,7 +43,7 @@ _ZFS_COMUTIL_H int zfs_spa_version_map(int zpl_version);
 _ZFS_COMUTIL_H boolean_t zfs_dataset_name_hidden(const char *);
 
 #define	ZFS_NUM_LEGACY_HISTORY_EVENTS 41
-_ZFS_COMUTIL_H const char *
+_ZFS_COMUTIL_H const char *const
     zfs_history_event_names[ZFS_NUM_LEGACY_HISTORY_EVENTS];
 
 #ifdef	__cplusplus