Btrfs: fix two use-after-free bugs with transaction cleanup

I was noticing the slab redzone stuff going off every once and a while during
transaction aborts.  This was caused by two things

1) We would walk the pending snapshots and set their error to -ECANCELED.  We
don't need to do this, the snapshot stuff waits for a transaction commit and if
there is a problem we just free our pending snapshot object and exit.  Doing
this was causing us to touch the pending snapshot object after the thing had
already been freed.

2) We were freeing the transaction manually with wanton disregard for it's
use_count reference counter.  To fix this I cleaned up the transaction freeing
loop to either wait for the transaction commit to finish if it was in the middle
of that (since it will be cleaned and freed up there) or to do the cleanup
oursevles.

I also moved the global "kill all things dirty everywhere" stuff outside of the
transaction cleanup loop since that only needs to be done once.  With this patch
I'm no longer seeing slab corruption because of use after frees.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>