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

module/os/linux/spl/spl-generic.c

@@ -44,17 +44,14 @@
 #include <sys/random.h>
 #include <sys/strings.h>
 #include <linux/kmod.h>
-#include "zfs_gitrev.h"
 #include <linux/mod_compat.h>
 #include <sys/cred.h>
 #include <sys/vnode.h>
 
-char spl_gitrev[64] = ZFS_META_GITREV;
-
 /* BEGIN CSTYLED */
 unsigned long spl_hostid = 0;
 EXPORT_SYMBOL(spl_hostid);
-/* BEGIN CSTYLED */
+
 module_param(spl_hostid, ulong, 0644);
 MODULE_PARM_DESC(spl_hostid, "The system hostid.");
 /* END CSTYLED */
@@ -632,7 +629,7 @@ spl_getattr(struct file *filp, struct kstat *stat)
  *
  */
 
-char *spl_hostid_path = HW_HOSTID_PATH;
+static char *spl_hostid_path = HW_HOSTID_PATH;
 module_param(spl_hostid_path, charp, 0444);
 MODULE_PARM_DESC(spl_hostid_path, "The system hostid file (/etc/hostid)");