LU-2869 llite: extended attribute cache

LU-2869 llite: extended attribute cache

This patch implements an extended attribute cache for
a Lustre client. It is organized as a write-through
cache: reads are performed from cache, updates are sent
synchronously to the MDS. An additional inode bit
MDS_INODELOCK_XATTR is added to protect the cache.

Signed-off-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Xyratex-bug-id: MRP-57
Change-Id: I16aae894a3c2f62448722eeade822ee22b20efa0
Reviewed-on: http://review.whamcloud.com/5537
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>