When a process exits, it might spend a lot of time in close_files() closing 
thousand of files.

This is bad for latency, and in some cases can trigger 'BUG: soft lockup 
detected on CPU#0!'

Call Trace: <IRQ> <ffffffff8024dafa>{softlockup_tick+250}
       <ffffffff80236cd7>{update_process_times+87} 
<ffffffff80215ed3>{smp_local_timer_interrupt+35}
       <ffffffff80216411>{smp_apic_timer_interrupt+65} 
<ffffffff8020a866>{apic_timer_interrupt+98} <EOI>
       <ffffffff803a7100>{sock_destroy_inode+0} 
<ffffffff803e4b1f>{tcp_send_fin+207}
       <ffffffff803e4ac4>{tcp_send_fin+116} <ffffffff803d988a>{tcp_close+698}
       <ffffffff803f4a07>{inet_release+87} <ffffffff803a84e9>{sock_release+25}
       <ffffffff803a87f5>{sock_close+53} <ffffffff802767a8>{__fput+88}
       <ffffffff8027382d>{filp_close+93} 
<ffffffff8022ee2e>{put_files_struct+110}
       <ffffffff8023025a>{do_exit+650} 
<ffffffff80237bee>{__dequeue_signal+478}
       <ffffffff80230998>{do_group_exit+200} 
<ffffffff80239c6a>{get_signal_to_deliver+1178}
       <ffffffff80209d47>{sysret_signal+28} <ffffffff80209001>{do_signal+129}
       <ffffffff8028d623>{dput+35} <ffffffff803a92db>{sys_accept+443}
       <ffffffff80241eec>{add_wait_queue+28} <ffffffff802ec581>{__up_write+33}
       <ffffffff80209d47>{sysret_signal+28} 
<ffffffff8020a033>{ptregscall_common+103}

This patch makes sure a cond_resched() call is done every 32 (or 64) files 
closed. This also helps reducing number of files waiting in RCU queues for 
final freeing as call_rcu() might have called force_quiescent_state()

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>