MDEV-31331: Fix cut'n'paste variable name in Debian pre-inst script

There is unwanted cut'n'paste variable name in Debian pre-inst
script which causes:

df: '': No such file or directory
/var/lib/dpkg/tmp.ci/preinst: line 215: [: : integer expression expected

Rename variable to correct one and make check that that directory
or symlink really exists. If it does not then fail with error
and message.

debian/mariadb-server-10.6.preinst

@@ -224,14 +224,23 @@ then
   mkdir -Z $mysql_datadir
 fi
 
-# As preset blocksize of GNU df is 1024 then available bytes is $df_available_blocks * 1024
-# 4096 blocks is then lower than 4 MB
-df_available_blocks="$(LC_ALL=C BLOCKSIZE='' df --output=avail "$mysql_datadir" | tail -n 1)"
-if [ "$df_available_blocks" -lt "4096" ]
+# Check if MariaDB datadir is available if not fails.
+# There should be symlink or directory available or something will fail.
+if [ -d "$mysql_datadir" ] || [ -L "$mysql_datadir" ]
 then
-  echo "ERROR: There's not enough space in $mysql_datadir/" 1>&2
-  db_stop
-  exit 1
+    # As preset blocksize of GNU df is 1024 then available bytes is $df_available_blocks * 1024
+    # 4096 blocks is then lower than 4 MB
+    df_available_blocks="$(LC_ALL=C BLOCKSIZE='' df --output=avail "$mysql_datadir" | tail -n 1)"
+    if [ "$df_available_blocks" -lt "4096" ]
+    then
+      echo "ERROR: There's not enough space in $mysql_datadir/" 1>&2
+      db_stop
+      exit 1
+    fi
+else
+    echo "ERROR: There's no directory or symlink available: $mysql_datadir/" 1>&2
+    db_stop
+    exit 1    
 fi
 
 # Since the home directory was created before putting the user into