afs: destroy work queue on init failure

We can clean up the work queue on this error path.  This function is
called from afs_init().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>