xfs: factor out a xfs_validate_rt_geometry helper

Split the RT geometry validation in the early mount code into a
helper than can be reused by repair (from which this code was
apparently originally stolen anyway).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: u64 return value for calc_rbmblocks]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>