xfs: update max log size

Commit a6634fba3dec4a92f0a2c4e30c80b634c0576ad5 in xfsprogs increased the
maximum log size supported by mkfs.  Merged back the changes to xfs_fs.h
so the growfs enforced the same limit and the headers are in sync.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>