LU-6483 utils: add setxattr/smallwrite for mdsrate

LU-6483 utils: add setxattr/smallwrite for mdsrate

Add --setxattr option for mdsrate, the XATTR value length is
4000.

--setxattr can be set along with --create or --mknod, which will
setxattr after file creation, and it can also be set alone which
will setxattr on existed files.

Add --smallwrite option for mdsrate --create, which will write
4000 bytes after creation, this is used to compare with
data-on-mds.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I47b13c619a17d5764c85feee8d46c2b87eab33f6
Reviewed-on: http://review.whamcloud.com/14536
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>