Normally current->jouranl_info is cleared by commit_transaction.  For an
async snap or subvol creation, though, it runs in a work queue.  Clear
it in btrfs_commit_transaction_async() to avoid leaking a non-NULL
journal_info when we return to userspace.  When the actual commit runs in
the other thread it won't care that it's current->journal_info is already
NULL.
Signed-off-by: NSage Weil <sage@newdream.net>
Tested-by: NJim Schutt <jaschut@sandia.gov>
Signed-off-by: NChris Mason <chris.mason@oracle.com>