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,
 };

lib/libzutil/zutil_import.c

@@ -1777,8 +1777,7 @@ zpool_find_import(libpc_handle_t *hdl, importargs_t *iarg)
 
 
 nvlist_t *
-zpool_search_import(void *hdl, importargs_t *import,
-    const pool_config_ops_t *pco)
+zpool_search_import(void *hdl, importargs_t *import, pool_config_ops_t *pco)
 {
 	libpc_handle_t handle = { 0 };
 	nvlist_t *pools = NULL;
@@ -1821,7 +1820,7 @@ pool_match(nvlist_t *cfg, char *tgt)
 
 int
 zpool_find_config(void *hdl, const char *target, nvlist_t **configp,
-    importargs_t *args, const pool_config_ops_t *pco)
+    importargs_t *args, pool_config_ops_t *pco)
 {
 	nvlist_t *pools;
 	nvlist_t *match = NULL;

lib/libzutil/zutil_import.h

@@ -43,7 +43,7 @@ typedef struct libpc_handle {
 	boolean_t lpc_open_access_error;
 	boolean_t lpc_desc_active;
 	char lpc_desc[1024];
-	const pool_config_ops_t *lpc_ops;
+	pool_config_ops_t *lpc_ops;
 	void *lpc_lib_handle;
 } libpc_handle_t;