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/sa.c

@@ -141,7 +141,7 @@ static int sa_modify_attrs(sa_handle_t *hdl, sa_attr_type_t newattr,
     sa_data_op_t action, sa_data_locator_t *locator, void *datastart,
     uint16_t buflen, dmu_tx_t *tx);
 
-static const arc_byteswap_func_t sa_bswap_table[] = {
+static arc_byteswap_func_t sa_bswap_table[] = {
 	byteswap_uint64_array,
 	byteswap_uint32_array,
 	byteswap_uint16_array,