dlm: sanitize work_start() in lowcomms.c

The create_workqueue() returns NULL if failed rather than ERR_PTR().
Fix error checking and remove unnecessary variable 'error'.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: David Teigland <teigland@redhat.com>