fs/quota: erase unused but set variable warning

stable inclusion
from stable-v4.19.311
commit 1974c13019b97140149690cd622a9c6732c32124
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IA8B9H
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1974c13019b97140149690cd622a9c6732c32124



--------------------------------

[ Upstream commit 78bc3334 ]

Local variable *reserved* of remove_dquot_ref() is only used if
define CONFIG_QUOTA_DEBUG, but not ebraced in CONFIG_QUOTA_DEBUG
macro, which leads to unused-but-set-variable warning when compiling.

This patch ebrace it into CONFIG_QUOTA_DEBUG macro like what is done
in add_dquot_ref().

Signed-off-by: Jiang Biao <benbjiang@tencent.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Stable-dep-of: 179b8c97ebf6 ("quota: Fix rcu annotations of inode dquot pointers")
Signed-off-by: Sasha Levin <sashal@kernel.org>

Conflicts:
	fs/quota/dquot.c
[Because commit 7bce48f0 ("quota: simplify drop_dquot_ref()") was merged
before this commit.]
Signed-off-by: Baokun Li <libaokun1@huawei.com>