Previously Tx.done was being set in close, but in a Tx
rollback and Commit are the real closing methods,
and Tx.close is just a helper common to both. Prior to this
change a multiple rollback statements could be called, one
would enter close and begin closing it while the other was
still in rollback breaking it. Fix that by setting done
in rollback and Commit, not in Tx.close.

Fixes #18429

Change-Id: Ie274f60c2aa6a4a5aa38e55109c05ea9d4fe0223
Reviewed-on: https://go-review.googlesource.com/34716Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>