xfs: fix exporting with left over 64-bit inodes

We now support mounting and using filesystems with 64-bit inodes
even when not mounted with the inode64 option (which now only
controls if we allocate new inodes in that space or not).  Make sure
we always use large NFS file handles when exporting a filesystem
that may contain 64-bit inodes.  Note that this only affects newly
generated file handles, any outstanding 32-bit file handle is still
accepted.

[hch: the comment and commit log are mine, the rest is from a patch
 snipplet from Samuel]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>