kexec: clear crashk_res if all its memory has been released

If the resource of crashk_res has been released, it is better to clear
crashk_res.start and crashk_res.end.  Because 'end = start - 1' is not
reasonable, and in some places the test is based on crashk_res.end, not
resource_size(&crashk_res).

Link: https://lkml.kernel.org/r/20230527123439.772-4-thunder.leizhen@huawei.com


Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Cong Wang <amwang@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>