xfs: bump max fsgeom struct version

The latest version of the fs geometry structure is v5.  Bump this
constant so that xfs_db and mkfs calls to libxfs_fs_geometry will fill
out all the fields.

IOWs, this commit is a no-op for the kernel, but will be useful for
userspace reporting in later changes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>