MDEV-22258 Limit innodb_encryption_threads to 255

For no good reason, innodb_encryption_threads was limited to
4,294,967,295. Expectedly, the server would crash if such an
insane value was specified. Let us limit the maximum to 255.

The encryption threads are not doing much useful work.
They are basically only dirtying pages by performing
dummy writes via the redo log. The encryption key rotation
or the in-place addition or removal of encryption
will take place in the page cleaner.

In a quick test on a 20-core CPU (40 threads in total),
the sweet spot on an otherwise idle server seemed to be
innodb_encryption_threads=16 for the test
encryption.encrypt_and_grep. The new limit 255 should be
more than enough for even bigger servers.

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -826,7 +826,7 @@ VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	INT UNSIGNED
 VARIABLE_COMMENT	Number of threads performing background key rotation and scrubbing
 NUMERIC_MIN_VALUE	0
-NUMERIC_MAX_VALUE	4294967295
+NUMERIC_MAX_VALUE	255
 NUMERIC_BLOCK_SIZE	0
 ENUM_VALUE_LIST	NULL
 READ_ONLY	NO

storage/innobase/handler/ha_innodb.cc

@@ -19919,7 +19919,7 @@ static MYSQL_SYSVAR_UINT(encryption_threads, srv_n_fil_crypt_threads,
 			 "scrubbing",
 			 NULL,
 			 innodb_encryption_threads_update,
-			 srv_n_fil_crypt_threads, 0, UINT_MAX32, 0);
+			 0, 0, 255, 0);
 
 static MYSQL_SYSVAR_UINT(encryption_rotate_key_age,
 			 srv_fil_crypt_rotate_key_age,

storage/xtradb/handler/ha_innodb.cc

@@ -21198,7 +21198,7 @@ static MYSQL_SYSVAR_UINT(encryption_threads, srv_n_fil_crypt_threads,
 			 "scrubbing",
 			 NULL,
 			 innodb_encryption_threads_update,
-			 srv_n_fil_crypt_threads, 0, UINT_MAX32, 0);
+			 0, 0, 255, 0);
 
 static MYSQL_SYSVAR_UINT(encryption_rotate_key_age,
 			 srv_fil_crypt_rotate_key_age,