staging: lustre/obdclass/acl: remove unused variable

lustre_ext_acl_xattr_reduce_space() declares old_size, but never uses it
and gcc 5.x likes to complain about this, so remove it as it's not used
at all.

Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>