In net_conf_import, start a transaction when importing a single share.

In net_conf_import, start a transaction when importing a single share.

Commit d69c3db9d44ad5d9fd1f5d7a9499f3bd79ecfb47 caused the transaction
start to be conditional but the commit is still unconditional, so an
error occurs when importing a single share.

An alternate fix would be to return the transaction start to be
unconditional but then it would occur before other error checking.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Signed-off-by: Michael Adam <obnox@samba.org>
(cherry picked from commit 9a28b28314c6a76cf619fc5f1b676d1f4229e3e1)