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 <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Ingo Molnar <[email protected]>
Acked-by: Baoquan He <[email protected]>
Cc: Dave Young <[email protected]>
Cc: David Howells <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: [email protected]
Cc: "Paul E. McKenney" <[email protected]>
Cc: Petr Tesarik <[email protected]>
Cc: [email protected]
Cc: Ram Pai <[email protected]>
Cc: Sinan Kaya <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Thymo van Beers <[email protected]>
Cc: [email protected]
Cc: x86-ml <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Zhimin Gu <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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