Always allow non-user xattrs

Under Linux you may only disable USER xattrs.  The SECURITY,
SYSTEM, and TRUSTED xattr namespaces must always be available
if xattrs are supported by the filesystem.  The enforcement
of USER xattrs is performed in the zpl_xattr_user_* handlers.

Under Solaris there is only a single xattr namespace which
is managed globally.

module/zfs/zfs_vnops.c

@@ -1112,14 +1112,6 @@ zfs_lookup(struct inode *dip, char *nm, struct inode **ipp, int flags,
 	*ipp = NULL;
 
 	if (flags & LOOKUP_XATTR) {
-		/*
-		 * If the xattr property is off, refuse the lookup request.
-		 */
-		if (!(zsb->z_flags & ZSB_XATTR)) {
-			ZFS_EXIT(zsb);
-			return (EINVAL);
-		}
-
 		/*
 		 * We don't allow recursive attributes..
 		 * Maybe someday we will.