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/zfs/zap.c

@@ -76,7 +76,7 @@
  *    the zfs-specific implementation of the directory's st_size (which is
  *    the number of entries).
  */
-int zap_iterate_prefetch = B_TRUE;
+static int zap_iterate_prefetch = B_TRUE;
 
 int fzap_default_block_shift = 14; /* 16k blocksize */