kexec/kdump: make struct crash_mem available without CONFIG_CRASH_DUMP

struct crash_mem defined under include/linux/crash_core.h represents
a list of memory ranges. While it is used to represent memory ranges
for kdump kernel, it can also be used for other kind of memory ranges.
In fact, KEXEC_FILE_LOAD syscall in powerpc uses this structure to
represent reserved memory ranges and exclude memory ranges needed to
find the right memory regions to load kexec kernel. So, make the
definition of crash_mem structure available for !CONFIG_CRASH_DUMP
case too.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240226103010.589537-2-hbathini@linux.ibm.com