xenbus: fix xenbus_transaction_start() hang caused by double xenbus_transaction_end()

xenbus: fix xenbus_transaction_start() hang caused by double xenbus_transaction_end()

vbd_resize() up_read()'s xs_state.suspend_mutex twice in a row via
double xenbus_transaction_end() calls. The next down_read() in
xenbus_transaction_start() (at eg. the next resize attempt) hangs.

See RHBZ#618317.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>