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

module/zfs/zio_compress.c

@@ -49,7 +49,7 @@ unsigned long zio_decompress_fail_fraction = 0;
 /*
  * Compression vectors.
  */
-const zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS] = {
+zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS] = {
 	{"inherit",	0,	NULL,		NULL, NULL},
 	{"on",		0,	NULL,		NULL, NULL},
 	{"uncompressed", 0,	NULL,		NULL, NULL},