SGI UV cpumask: use static temp cpumask in flush_tlb

Impact: Improve tlb flush performance for UV

Calling alloc_cpumask_var a zillion times a second does affect
performance.  Replace with static cpumask.

Note: when CONFIG_X86_UV is defined, this extra PER_CPU memory
will be optimized out for non-UV configs as is_uv_system() will
then return a constant 0.

Signed-off-by: Mike Travis <travis@sgi.com>