xfs: fix nfs export of 64-bit inodes numbers on 32-bit kernels

The i_ino field in the VFS inode is of type unsigned long and thus can't
hold the full 64-bit inode number on 32-bit kernels.  We have the full
inode number in the XFS inode, so use that one for nfs exports.  Note
that I've also switched the 32-bit file handles types to it, just to make
the code more consistent and copy & paste errors less likely to happen.

Reported-by: Guoquan Yang <ygq51@hotmail.com>
Reported-by: Hank Peng <pengxihan@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>