f2fs: fix to allow removing qf_name

The mount behavior changed after commit d185351 ("f2fs: separate the
options parsing and options checking"), let's fix it.

[Scripts]
mkfs.f2fs -f /dev/vdb
mount -t f2fs -o usrquota /dev/vdb /mnt/f2fs
quotacheck -uc /mnt/f2fs
umount /mnt/f2fs
mount -t f2fs -o usrjquota=aquota.user,jqfmt=vfsold /dev/vdb /mnt/f2fs
mount|grep f2fs
mount -t f2fs -o remount,usrjquota=,jqfmt=vfsold /dev/vdb /mnt/f2fs
mount|grep f2fs
dmesg

[Before commit]
mount#1: ...,quota,jqfmt=vfsold,usrjquota=aquota.user,...
mount#2: ...,quota,jqfmt=vfsold,...
kmsg: no output

[After commit]
mount#1: ...,quota,jqfmt=vfsold,usrjquota=aquota.user,...
mount#2: ...,quota,jqfmt=vfsold,usrjquota=aquota.user,...
kmsg: "user quota file already specified"

[After patch]
mount#1: ...,quota,jqfmt=vfsold,usrjquota=aquota.user,...
mount#2: ...,quota,jqfmt=vfsold,...
kmsg: "remove qf_name aquota.user"

Fixes: d185351 ("f2fs: separate the options parsing and options checking")
Cc: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

fs/f2fs/super.c

@@ -1219,8 +1219,11 @@ static int f2fs_check_quota_consistency(struct fs_context *fc,
 				goto err_jquota_change;
 
 			if (old_qname) {
-				if (new_qname &&
-					strcmp(old_qname, new_qname) == 0) {
+				if (!new_qname) {
+					f2fs_info(sbi, "remove qf_name %s",
+								old_qname);
+					continue;
+				} else if (strcmp(old_qname, new_qname) == 0) {
 					ctx->qname_mask &= ~(1 << i);
 					continue;
 				}