blktap: fix cleanup after unclean application exit #2

blktap: fix cleanup after unclean application exit #2

When an application using blktap devices doesn't close the mmap-s of
/dev/xen/blktapN and the frontend driver never connects, we cannot
defer the mmput() on the stored mm until blktap_release() or the exit
path of the worker thread, as the former will never be called without
the mm's reference count dropping to zero, and the worker thread
would never get started.

Also remove a left-over from c/s 1013.

Signed-off-by: Jan Beulich <jbeulich@novell.com>