x86/platform/UV: Fix panic with missing UVsystab support

x86/platform/UV: Fix panic with missing UVsystab support

Fix the panic where KEXEC'd kernel does not have access to EFI runtime
mappings.  This may cause the extended UVsystab to not be available.
The solution is to revert to non-UV mode and continue with limited
capabilities.

Signed-off-by: Mike Travis <travis@sgi.com>
Reviewed-by: Russ Anderson <rja@hpe.com>
Reviewed-by: Alex Thorlton <athorlton@sgi.com>
Acked-by: Dimitri Sivanich <sivanich@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170113152111.118886202@asylum.americas.sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>