staging: lustre: llite: add simple comment about lustre.lov xattrs

Simple comment added to ll_xattr_set.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8998
Reviewed-on: https://review.whamcloud.com/24851


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>