x86/kdump: Fall back to reserve high crashkernel memory

crashkernel=xM tries to reserve memory for the crash kernel under 4G,
which is enough, usually. But this could fail sometimes, for example
when one tries to reserve a big chunk like 2G, for example.

So let the crashkernel=xM just fall back to use high memory in case it
fails to find a suitable low range. Do not set the ,high as default
because it allocates extra low memory for DMA buffers and swiotlb, and
this is not always necessary for all machines.

Typically, crashkernel=128M usually works with low reservation under 4G,
so keep <4G as default.

 [ bp: Massage. ]

Signed-off-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Eric 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: Jiri Kosina <jkosina@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: linux-doc@vger.kernel.org
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Petr Tesarik <ptesarik@suse.cz>
Cc: piliu@redhat.com
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Sinan Kaya <okaya@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thymo van Beers <thymovanbeers@gmail.com>
Cc: vgoyal@redhat.com
Cc: x86-ml <x86@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Zhimin Gu <kookoo.gu@intel.com>
Link: https://lkml.kernel.org/r/20190422031905.GA8387@dhcp-128-65.nay.redhat.com