f2fs: fix to allow removing qf_name

The mount behavior changed after commit d1853513 ("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: d1853513 ("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>