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/vdev_impl.h

@@ -626,8 +626,6 @@ extern uint64_t vdev_get_ndisks(vdev_t *vd);
  * Global variables
  */
 extern int zfs_vdev_standard_sm_blksz;
-/* zdb uses this tunable, so it must be declared here to make lint happy. */
-extern int zfs_vdev_cache_size;
 
 /*
  * Functions from vdev_indirect.c