fuse: conditionally fill kstat in fuse_do_statx()

The code path

fuse_update_attributes
    fuse_update_get_attr
        fuse_do_statx

has the risk to use a NULL pointer for struct kstat *stat, although current
callers of fuse_update_attributes() only set request_mask to values that
will trigger the call of fuse_do_getattr(), which already handles the NULL
pointer.  Future updates might miss that fuse_do_statx() does not handle it
it is safer to add a condition already right now.
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Fixes: d3045530 ("fuse: implement statx")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -1219,11 +1219,14 @@ static int fuse_do_statx(struct inode *inode, struct file *file,
 		fuse_change_attributes(inode, &attr, &outarg.stat,
 				       ATTR_TIMEOUT(&outarg), attr_version);
 	}
-	stat->result_mask = sx->mask & (STATX_BASIC_STATS | STATX_BTIME);
-	stat->btime.tv_sec = sx->btime.tv_sec;
-	stat->btime.tv_nsec = min_t(u32, sx->btime.tv_nsec, NSEC_PER_SEC - 1);
-	fuse_fillattr(inode, &attr, stat);
-	stat->result_mask |= STATX_TYPE;
+
+	if (stat) {
+		stat->result_mask = sx->mask & (STATX_BASIC_STATS | STATX_BTIME);
+		stat->btime.tv_sec = sx->btime.tv_sec;
+		stat->btime.tv_nsec = min_t(u32, sx->btime.tv_nsec, NSEC_PER_SEC - 1);
+		fuse_fillattr(inode, &attr, stat);
+		stat->result_mask |= STATX_TYPE;
+	}
 
 	return 0;
 }