quota: Convert dqio_mutex to rwsem

Convert dqio_mutex to rwsem and call it dqio_sem. No functional changes
yet.

Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/quota/quota_tree.c b/fs/quota/quota_tree.c
index 0738972..54f85eb 100644
--- a/fs/quota/quota_tree.c
+++ b/fs/quota/quota_tree.c
@@ -379,7 +379,7 @@ int qtree_write_dquot(struct qtree_mem_dqinfo *info, struct dquot *dquot)
 	if (!ddquot)
 		return -ENOMEM;
 
-	/* dq_off is guarded by dqio_mutex */
+	/* dq_off is guarded by dqio_sem */
 	if (!dquot->dq_off) {
 		ret = dq_insert_tree(info, dquot);
 		if (ret < 0) {