md-cache: fix xattr caching code in getxattr

Bad condition check, fix it!

Change-Id: I6e047de70f77d7b98b2ca771a467f14a76fd62fe
BUG: 994392
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5513
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>