Btrfs: fix transaction abortion when remounting btrfs from RW to RO

Btrfs: fix transaction abortion when remounting btrfs from RW to RO

Steps to reproduce:
 # mkfs.btrfs -f /dev/sda8
 # mount /dev/sda8 /mnt -o flushoncommit
 # dd if=/dev/zero of=/mnt/data bs=4k count=102400 &
 # mount /dev/sda8 /mnt -o remount, ro

When remounting RW to RO, the logic is to firstly set flag
to RO and then commit transaction, however with option
flushoncommit enabled,we will do RO check within committing
transaction, so we get a transaction abortion here.

Actually,here check is wrong, we should check if FS_STATE_ERROR
is set, fix it.

Reported-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Suggested-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>