crash_dump: store dm crypt keys in kdump reserved memory

When the kdump kernel image and initrd are loaded, the dm crypts keys will
be read from keyring and then stored in kdump reserved memory.

Assume a key won't exceed 256 bytes thus MAX_KEY_SIZE=256 according to
"cryptsetup benchmark".

Link: https://lkml.kernel.org/r/20250502011246.99238-4-coxu@redhat.com


Signed-off-by: Coiby Xu <coxu@redhat.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Jan Pazdziora <jpazdziora@redhat.com>
Cc: Liu Pingfan <kernelfans@gmail.com>
Cc: Milan Broz <gmazyland@gmail.com>
Cc: Ondrej Kozina <okozina@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>