btrfs: bail out of uuid tree scanning if we're closing

In doing my fsstress+EIO stress testing I started running into issues
where umount would get stuck forever because the uuid checker was
chewing through the thousands of subvolumes I had created.  We shouldn't
block umount on this, simply bail if we're unmounting the fs.  We need
to make sure we don't mark the UUID tree as ok, so we only set that bit
if we made it through the whole rescan operation, but otherwise this is
completely safe.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>