sparc64: Skip flushing TLBs if there are no mm_users

sparc64: Skip flushing TLBs if there are no mm_users

Orabug: 25379970

Saves time when smp_flush_tlb_page/smp_flush_tlb_pending
is called during do_exit(...). Without this patch, killing
processes had performance bottle neck in these functions
due to unnecessary xcalls made to flush TLBs.

Reviewed-by: Nitin Gupta <nitin.m.gupta@oracle.com>
Signed-off-by: Bob Picco <bob.picco@oracle.com
Signed-off-by: Henry Willard <henry.willard@oracle.com>
Signed-off-by: Sanath Kumar <sanath.s.kumar@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>