kexec: delete a useless check in crash_shrink_memory()

The check '(crashk_res.parent != NULL)' is added by commit e05bd336
("kexec: fix Oops in crash_shrink_memory()"), but it's stale now.  Because
if 'crashk_res' is not reserved, it will be zero in size and will be
intercepted by the above 'if (new_size >= old_size)'.

Ago:
	if (new_size >= end - start + 1)

Now:
	old_size = (end == 0) ? 0 : end - start + 1;
	if (new_size >= old_size)

Link: https://lkml.kernel.org/r/20230527123439.772-3-thunder.leizhen@huawei.com


Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Cc: 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>