LU-10792 llite: remove unused parameters from md_{get,set}xattr()

LU-10792 llite: remove unused parameters from md_{get,set}xattr()

md_getxattr() and md_setxattr() each have several unused
parameters. Remove them and improve the naming or remaining
parameters.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I578bdd5dab70745ba7f8fbb9f047fa9eb1f6ee9a
Reviewed-on: https://review.whamcloud.com/31592
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>