staging/lustre/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.

Lustre-change: http://review.whamcloud.com/5537
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2869


Signed-off-by: Andrew Perepechko <andrew_perepechko@xyratex.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>
[remove extra GPL notice in original patch as kernel already has one
and it causes checkpatch error. -- Peng Tao]
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>