LU-3270 statahead: small fixes and cleanup

LU-3270 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>
Change-Id: I65759c7dfcbe879b42f14152dbfe5949e3d37ea0
Reviewed-on: http://review.whamcloud.com/9667
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>