Btrfs: fix race during transaction joins

Btrfs: fix race during transaction joins

While we're allocating ram for a new transaction, we drop our spinlock.
When we get the lock back, we do check to see if a transaction started
while we slept, but we don't check to make sure it isn't blocked
because a commit has already started.

Signed-off-by: Chris Mason <chris.mason@oracle.com>