libceph: create_singlethread_workqueue() doesn't return ERR_PTRs

commit dbcae088fa660086bde6e10d63bb3c9264832d85 upstream.

create_singlethread_workqueue() returns NULL on error, and it doesn't
return ERR_PTRs.

I tweaked the error handling a little to be consistent with earlier in
the function.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>