Btrfs: add tree block level sanity check

With a users corrupted fs I was getting weird behavior and panics and it turns
out it was because one of his tree blocks had a bogus header level.  So add this
to the sanity checks in the endio handler for tree blocks.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>

diff --cc fs/btrfs/disk-io.c
Simple merge