xfs: make XFS_DQUOT_CLUSTER_SIZE_FSB part of the ondisk format

Move the dquot cluster size #define to xfs_format.h.  It is an important
part of the ondisk format because the ondisk dquot record size is not an
even power of two, which means that the buffer size we use is
significant here because the kernel leaves slack space at the end of the
buffer to avoid having to deal with a dquot record crossing a block
boundary.

This is also an excuse to fix one of the longstanding discrepancies
between kernel and userspace libxfs headers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>