afs: Fix afs_atcell_get_link() to check if ws_cell is unset first

Fix afs_atcell_get_link() to check if the workstation cell is unset before
doing the RCU pathwalk bit where we dereference that.

Fixes: 823869e1 ("afs: Fix afs_atcell_get_link() to handle RCU pathwalk")
Reported-by:  <syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/2481796.1742296819@warthog.procyon.org.uk


Tested-by:  <syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>