MDEV-15925 FRM_MAX_SIZE too low for some use cases

increase to 1M

mysql-test/r/huge_frm-6224.result

+set global max_allowed_packet=1024*1024*10;
 ERROR HY000: The definition for table `t1` is too big
+set global max_allowed_packet=default;

mysql-test/t/huge_frm-6224.test

@@ -4,17 +4,24 @@
 # verify that huge frms are rejected during creation, not on opening
 #
 --source include/have_partition.inc
+set global max_allowed_packet=1024*1024*10;
+connect con1,localhost,root;
 
-let $n=5646;
+let $n=8164;
 let $a=create table t1 (a int) engine=myisam partition by hash(a) partitions $n (;
 dec $n;
 while ($n)
 {
-  let $a=$a partition p01234567890123456789012345678901234567890123456789012345678$n,;
+  let $a=$a partition p01234567890123456789012345678901234567890123456789012345678$n COMMENT 'partition p01234567890123456789012345678901234567890123456789012345678$n',;
   dec $n;
 }
 
 --disable_query_log
 --error ER_TABLE_DEFINITION_TOO_BIG
 eval $a partition foo);
+--enable_query_log
+
+connection default;
+disconnect con1;
+set global max_allowed_packet=default;
 

sql/unireg.h

@@ -202,7 +202,7 @@ LEX_CUSTRING build_frm_image(THD *thd, const char *table,
 
 #define FRM_HEADER_SIZE 64
 #define FRM_FORMINFO_SIZE 288
-#define FRM_MAX_SIZE (512*1024)
+#define FRM_MAX_SIZE (1024*1024)
 
 static inline bool is_binary_frm_header(uchar *head)
 {