xfs: Move extent count limits to xfs_format.h

Maximum values associated with extent counters i.e. Maximum extent length,
Maximum data extents and Maximum xattr extents are dictated by the on-disk
format. Hence move these definitions over to xfs_format.h.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>