quota: check quota reservation on remove_dquot_ref

Reserved space must being claimed before remove_dquot_ref() for a
given inode. Filesystem is responsible for performing force blocks
allocation in case of dealloc in ->quota_off. Let's add sanity check
for that case. Do it similar to add_dquot_ref().

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>