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

In commit f99a891 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.

Author: ottok

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

@@ -211,7 +211,7 @@ 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
  db_stop