staging: lustre: statahead: small fixes and cleanup

small fixes:
 * when 'unplug' is set for ll_statahead(), sa_put() shouldn't kill
   the entry found, because its inflight RPC may not finish yet.
 * remove 'sai_generation', add 'lli_sa_generation' because the
   former one is not safe to access without lock.
 * revalidate_statahead_dentry() may fail to wait for statahead
   entry to become ready, in this case it should not release this
   entry, because it may be used by inflight statahead RPC.

cleanups:
 * rename ll_statahead_enter() to ll_statahead().
 * move dentry 'lld_sa_generation' update to ll_statahead() to
   simplify code and logic.
 * other small cleanups.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3270
Reviewed-on: http://review.whamcloud.com/9667
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6222
Reviewed-on: http://review.whamcloud.com/13708


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>