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/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;