When CPU or memory is hot un/plugged, or off/onlined, the crash
elfcorehdr, which describes the CPUs and memory in the system, must also
be updated.

A new elfcorehdr is generated from the available CPUs and memory and
replaces the existing elfcorehdr.  The segment containing the elfcorehdr
is identified at run-time in crash_core:crash_handle_hotplug_event().

No modifications to purgatory (see 'kexec: exclude elfcorehdr from the
segment digest') or boot_params (as the elfcorehdr= capture kernel command
line parameter pointer remains unchanged and correct) are needed, just
elfcorehdr.

For kexec_file_load(), the elfcorehdr segment size is based on NR_CPUS and
CRASH_MAX_MEMORY_RANGES in order to accommodate a growing number of CPU
and memory resources.

For kexec_load(), the userspace kexec utility needs to size the elfcorehdr
segment in the same/similar manner.

To accommodate kexec_load() syscall in the absence of kexec_file_load()
syscall support, prepare_elf_headers() and dependents are moved outside of
CONFIG_KEXEC_FILE.

[eric.devolder@oracle.com: correct unused function build error]
  Link: https://lkml.kernel.org/r/20230821182644.2143-1-eric.devolder@oracle.com
Link: https://lkml.kernel.org/r/20230814214446.6659-6-eric.devolder@oracle.comSigned-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Akhil Raj <lf32.dev@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Weißschuh <linux@weissschuh.net>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>