x86/UV: Check for alloc_cpumask_var() failures properly in uv_nmi_setup()

x86/UV: Check for alloc_cpumask_var() failures properly in uv_nmi_setup()

GCC warned about:

   arch/x86/platform/uv/uv_nmi.c: In function ‘uv_nmi_setup’:
   arch/x86/platform/uv/uv_nmi.c:664:2: warning: the address of ‘uv_nmi_cpu_mask’ will always evaluate as ‘true’

The reason is this code:

        alloc_cpumask_var(&uv_nmi_cpu_mask, GFP_KERNEL);
        BUG_ON(!uv_nmi_cpu_mask);

which is not the way to check for alloc_cpumask_var() failures - its
return code should be checked instead.

Cc: Mike Travis <travis@sgi.com>
Link: http://lkml.kernel.org/n/tip-2pXRemsjupmvonbpmmnzleo1@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>