Btrfs: protect the pending_snapshots list with trans_lock

Currently there is nothing protecting the pending_snapshots list on the
transaction.  We only hold the directory mutex that we are snapshotting and a
read lock on the subvol_sem, so we could race with somebody else creating a
snapshot in a different directory and end up with list corruption.  So protect
this list with the trans_lock.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>