GFS2: fix error propagation in init_threads()

If kthread_run() fails, init_threads() returns
IS_ERR(p) instead of PTR_ERR(p).

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>