staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-10787
Reviewed-on: https://review.whamcloud.com/


Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>