Comment # 5 on bug 1231836 from Wenruo Qu

The direct cause is, when the initial super RO flag mismatches, we got EBUSY
and go btrfs_reconfigure_for_mount(), which flips our RO flag and retry.

But since the background process is also re-mounting which can flips the RO
flag to a different one, we got -EBUSY again because the newly flipped RO flag
conflicts with the newly remounted flag.

This involves a lot of VFS calls which can be a little complex, but I'll try if
we can do a mutex or something like that to avoid remount/mount to race on the
same btrfs.