Btrfs corruption fix

Hi Linus,

I'm testing a pull with more fixes, but wanted to get this one out so Greg can pick it up. The corruption isn't easy to hit, you have to do a readonly snapshot and have orphans in the snapshot. But my review and testing missed the bug. Filipe has added a better xfstest to cover it.

Please pull:

git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs.git for-linus

Chris Mason (1) commits (+33/-36):
Revert "Btrfs: race free update of commit root for ro snapshots"

Total: (1) commits (+33/-36)

 fs/btrfs/inode.c | 36 ------------------------------------
 fs/btrfs/ioctl.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 36 deletions(-)

Sorry Linus, I realized this morning I forgot my sob. I've pushed out an updated commit to a new for-linus-update branch. The code and diffstat are exactly the same:

git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs.git for-linus-update

-chris