xfs: Make inode32 a remountable option

As inode64 is the default option now, and was also made remountable
previously, inode32 can also be remounted on-the-fly when it is needed.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>