ksmbd: fix possible null-deref in smb_lazy_parent_lease_break_close

[ Upstream commit 5fb282ba

 ]

rcu_dereference can return NULL, so make sure we check against that.

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>