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

include/sys/dmu.h

@@ -907,7 +907,7 @@ typedef const struct dmu_object_byteswap_info {
 } dmu_object_byteswap_info_t;
 
 extern const dmu_object_type_info_t dmu_ot[DMU_OT_NUMTYPES];
-extern const dmu_object_byteswap_info_t dmu_ot_byteswap[DMU_BSWAP_NUMFUNCS];
+extern dmu_object_byteswap_info_t dmu_ot_byteswap[DMU_BSWAP_NUMFUNCS];
 
 /*
  * Get information on a DMU object.

include/sys/zio_checksum.h

@@ -102,7 +102,7 @@ typedef struct zio_bad_cksum {
 	uint8_t			zbc_has_cksum;	/* expected/actual valid */
 } zio_bad_cksum_t;
 
-_SYS_ZIO_CHECKSUM_H const zio_checksum_info_t
+_SYS_ZIO_CHECKSUM_H zio_checksum_info_t
     zio_checksum_table[ZIO_CHECKSUM_FUNCTIONS];
 
 /*

include/sys/zio_compress.h

@@ -152,7 +152,7 @@ typedef const struct zio_compress_info {
 	zio_decompresslevel_func_t	*ci_decompress_level;
 } zio_compress_info_t;
 
-extern const zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS];
+extern zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS];
 
 /*
  * lz4 compression init & free