LU-5682 lfsck: optimize ldlm lock used by LFSCK

LU-5682 lfsck: optimize ldlm lock used by LFSCK

When LFSCK repairs some inconsistency, it needs to take related
ldlm lock(s) firstly to prevent concurrent modifications or purge
client side cache. Originally, to simply the implementation, the
LFSCK just simply acquires LCK_EX mode ibits lock(s) on related
object(s). But such coarse-grained lock policy may be not efficient
for some directory-based modification, such as insert name entry to
the directory.

This patch introduces lfsck PDO (Parallel Directory Operations) lock
for directory-based LFSCK modification, it only locks part of the
directory with the given <object, name> pairs, then allow others to
access or modify the different part(s) of the directory in parallel,
and also avoid to purge client-side cache unnecessarily.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I29bad81112c14e3aaecaa2b808e60ea74c10a702
Reviewed-on: http://review.whamcloud.com/12766
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>