LU-9856 mdd: handle NULL buffer in mdd_xattr_list()

LU-9856 mdd: handle NULL buffer in mdd_xattr_list()

The upper layer may call mdd_xattr_list() with a NULL buffer to get
the length of the xattr name list. Handle this case safely by skipping
the removal of the link xattr for unlinked objects.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Iae87fba20325b228ef75ee762acfa49353932b1b
Reviewed-on: https://review.whamcloud.com/28469
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>