[PATCH] Retrieve MAX_PHYSMEM_BITS from vmcoreinfo

Add a common feature for architectures (except arm64, for which
similar support is added via a subsequent patch) to retrieve
MAX_PHYSMEM_BITS from vmcoreinfo, which was added by kernel commit
1d50e5d0c505 ("crash_core, vmcoreinfo: Append 'MAX_PHYSMEM_BITS'
to vmcoreinfo").  This makes makedumpfile adaptable for future
MAX_PHYSMEM_BITS changes.

Also ensure backward compatibility for kernel versions in which
MAX_PHYSMEM_BITS is not available in vmcoreinfo.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>