LU-10787 llite: correct removexattr detection

LU-10787 llite: correct removexattr detection

In ll_xattr_set_common() detect the removexattr() case correctly by
testing for a NULL value as well as XATTR_REPLACE.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I29a29851ad4ac432e257b63088e2d7a7dfc39605
Reviewed-on: https://review.whamcloud.com/31594
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>