[XFS] allow inode64 mount option on 32 bit systems

Now that we've stopped using the Linux inode cache when can trivally
support the inode64 mount option on 32bit architectures.  As far as the
kernel and most userspace is concerned this works perfectly, but
applications still using really old stat and readdir interfaces will get
an EOVERFLOW error when hitting an inode number not fitting into 32
bits (that problem of course also exists when using these applications
on a 64bit kernel).

Note that because inode64 is simply a mount option we can currently
mount a filesystem having > 32 bit inode numbers and cause a variety of
problems, all this is solved but this patch which enables XFS_BIG_INUMS,
even when inode64 is not used.

(First sent on October 18th)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>