[SPARC64]: Disable preemption during flush_tlb_pending().

A context switch will force a call to flush_tlb_pending() (via
switch_to()), so if we test tlb_nr to be non-zero, then sleep, it
would become zero and later back at the original context we'll pass
zero down into the TLB flushing code which should never see a nr
argument of zero.

Signed-off-by: David S. Miller <davem@davemloft.net>