Allow one to decrease page-buffer-size down to 1M (from 128M) for maria_read_log

Don't allow too low value of pagecache_buffer_size for mysqld

storage/maria/ha_maria.cc:
  Don't allow too low value of pagecache_buffer_size (as it will stop mysqld from starting)
storage/maria/maria_read_log.c:
  Allow one to decrease page-buffer-size down to 1M (from 128M)

storage/maria/ha_maria.cc

@@ -191,7 +191,7 @@ static MYSQL_SYSVAR_ULONGLONG(pagecache_buffer_size, pagecache_buffer_size,
        "The size of the buffer used for index blocks for Maria tables. "
        "Increase this to get better index handling (for all reads and "
        "multiple writes) to as much as you can afford.", 0, 0,
-       KEY_CACHE_SIZE, MALLOC_OVERHEAD, ~(ulong) 0, IO_SIZE);
+       KEY_CACHE_SIZE, 8192*16L, ~(ulong) 0, IO_SIZE);
 
 static MYSQL_SYSVAR_ULONG(pagecache_division_limit, pagecache_division_limit,
        PLUGIN_VAR_RQCMDARG,

storage/maria/maria_read_log.c

@@ -199,7 +199,8 @@ static struct my_option my_long_options[] =
     "Path to the directory where to store transactional log",
     (uchar **) &maria_data_root, (uchar **) &maria_data_root, 0,
     GET_STR, REQUIRED_ARG, 0, 0, 0, 0, 0, 0},
-  { "page-buffer-size", 'P', "",
+  { "page-buffer-size", 'P',
+    "The size of the buffer used for index blocks for Maria tables",
     &opt_page_buffer_size, &opt_page_buffer_size, 0,
     GET_ULONG, REQUIRED_ARG, (long) USE_BUFFER_INIT,
     1024L*1024L, (long) ~(ulong) 0, (long) MALLOC_OVERHEAD,