xfs: use xfs_vn_setattr_size to check on new size

Commit 65523218 ("xfs: remove i_iolock and use i_rwsem in the
VFS inode instead") introduced a regression that truncate(2) doesn't
check on new size, so it succeeds even if the new size exceeds the
current resource limit. Because xfs_setattr_size() was used instead
of xfs_vn_setattr_size(), and the latter calls xfs_vn_change_ok()
first to do sanity check on permission and new size.

This is found by truncate03 test from ltp, and the following is a
simplified reproducer:

  #!/bin/bash
  dev=/dev/sda5
  mnt=/mnt/xfs

  mkfs -t xfs -f $dev
  mount $dev $mnt

  # set max file size to 16k
  ulimit -f 16
  truncate -s $((16 * 1024 + 1)) /mnt/xfs/testfile
  [ $? -eq 0 ] && echo "FAIL: truncate exceeded max file size"
  ulimit -f unlimited
  umount $mnt

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>