We have lots of places that want to calculate the final fsb for
a offset + count in bytes and check that the result fits into
s_maxbytes.  Factor out a helper for that.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NAllison Collins <allison.henderson@oracle.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>