quota: Fix quota corruption with generic/232 test

Eric has reported that since commit d2faa415166b "quota: Do not acquire
dqio_sem for dquot overwrites in v2 format" test generic/232
occasionally fails due to quota information being incorrect. Indeed that
commit was too eager to remove dqio_sem completely from the path that
just overwrites quota structure with updated information. Although that
is innocent on its own, another process that inserts new quota structure
to the same block can perform read-modify-write cycle of that block thus
effectively discarding quota information update if they race in a wrong
way.

Fix the problem by acquiring dqio_sem for reading for overwrites of
quota structure. Note that it *is* possible to completely avoid taking
dqio_sem in the overwrite path however that will require modifying path
inserting / deleting quota structures to avoid RMW cycles of the full
block and for now it is not clear whether it is worth the hassle.

Fixes: d2faa415166b2883428efa92f451774ef44373ac
Reported-and-tested-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/quota_v2.c

@@ -328,12 +328,16 @@ static int v2_write_dquot(struct dquot *dquot)
 	if (!dquot->dq_off) {
 		alloc = true;
 		down_write(&dqopt->dqio_sem);
+	} else {
+		down_read(&dqopt->dqio_sem);
 	}
 	ret = qtree_write_dquot(
 			sb_dqinfo(dquot->dq_sb, dquot->dq_id.type)->dqi_priv,
 			dquot);
 	if (alloc)
 		up_write(&dqopt->dqio_sem);
+	else
+		up_read(&dqopt->dqio_sem);
 	return ret;
 }