LU-11695 som: disabling xattr cache for LSOM on client

LU-11695 som: disabling xattr cache for LSOM on client

To obtain uptodate LSOM data, currently a client needs to set
llite.*.xattr_cache =0 to disable the xattr cache on client
completely. This leads that other kinds of xattr can not be cached
on the client too.
This patch introduces a heavy-weight solution to disable caching
only for LSOM xattr data ("trusted.som") on client.

Lustre-change: https://review.whamcloud.com/33711/
Lustre-commit: TBD (from 55a65276b8e62f0aa894ee0719051ed8737ab59e)

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Iab5ef3030b05ac09184d01f2a3a8ed92ff1cf26b
Reviewed-on: https://review.whamcloud.com/47813
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>