LU-9727 lustre: add CL_GETXATTR for Changelogs

LU-9727 lustre: add CL_GETXATTR for Changelogs

Record GETXATTR events in Changelogs, and add a new changelog
extension named changelog_ext_xattr to hold xattr name.
A GETXATTR changlog entry is in the following form:
8 23GXATR 09:22:55.886793012 2017.07.27 0x0
t=[0x200000402:0x1:0x0] ef=0xf u=500:500 nid=10.128.11.159@tcp
x=user.name0
Also, rename CL_XATTR type to CL_SETXATTR.
By default, disable recording of GETXATTR events in Changelogs.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ia02e870ca162c7d2b97eb0ce80e99fe7145b7601
Reviewed-on: https://review.whamcloud.com/28251
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>