Deb: Fix blocksize check to use $mysql_datadir/$datadir correctly

In commit f99a8918 this line was changed to not use awk, and new version
copied both to init file and preinst file but overlooking that they use
different variable names.

Also fix minor syntax issues to make Shellcheck happy.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.

debian/mariadb-server-10.6.mariadb.init

@@ -86,7 +86,7 @@ sanity_checks() {
   datadir=`mariadbd_get_param datadir`
   # As preset blocksize of GNU df is 1024 then available bytes is $df_available_blocks * 1024
   # 4096 blocks is then lower than 4 MB
-  df_available_blocks=`LC_ALL=C BLOCKSIZE= df --output=avail "$datadir" | tail -n 1`
+  df_available_blocks="$(LC_ALL=C BLOCKSIZE='' df --output=avail "$datadir" | tail -n 1)"
   if [ "$df_available_blocks" -lt "4096" ]; then
     log_failure_msg "$0: ERROR: The partition with $datadir is too full!"
     echo                "ERROR: The partition with $datadir is too full!" | $ERR_LOGGER

debian/mariadb-server-10.6.preinst

@@ -226,7 +226,7 @@ fi
 
 # As preset blocksize of GNU df is 1024 then available bytes is $df_available_blocks * 1024
 # 4096 blocks is then lower than 4 MB
-df_available_blocks=`LC_ALL=C BLOCKSIZE= df --output=avail "$datadir" | tail -n 1`
+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