Fix double const qualifier declarations

Some header files define structures like this one:

typedef const struct zio_checksum_info {
	/* ... */
	const char	*ci_name;
} zio_abd_checksum_func_t;

So we can use `zio_abd_checksum_func_t` for const declarations now.
It's not needed that we use the `const` qualifier again like this:
`const zio_abd_checksum_func_t *varname;`

This patch solves the double const qualifiers, which were found by
smatch.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13961

lib/libzpool/util.c

@@ -350,7 +350,7 @@ pool_active(void *unused, const char *name, uint64_t guid,
 }
 #endif
 
-const pool_config_ops_t libzpool_config_ops = {
+pool_config_ops_t libzpool_config_ops = {
 	.pco_refresh_config = refresh_config,
 	.pco_pool_active = pool_active,
 };