gfs2: Fix use-after-free in gfs2_glock_shrink_scan

stable inclusion
from stable-v4.19.196
commit 094bf5670e762afa243d2c41a5c4ab71c7447bf4
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9R4CX
CVE: CVE-2021-47254

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=094bf5670e762afa243d2c41a5c4ab71c7447bf4



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[ Upstream commit 1ab19c5d ]

The GLF_LRU flag is checked under lru_lock in gfs2_glock_remove_from_lru() to
remove the glock from the lru list in __gfs2_glock_put().

On the shrink scan path, the same flag is cleared under lru_lock but because
of cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the
put side can be made without deleting the glock from the lru list.

Keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) to
ensure correct behavior on both sides - clear GLF_LRU after list_del under
lru_lock.

Reported-by: syzbot <syzbot+34ba7ddbf3021981a228@syzkaller.appspotmail.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zhao Wenhui <zhaowenhui8@huawei.com>