xfs: cleanup xfs_fsb_to_db

This function isn't a macro anymore, so remove various superflous braces,
and explicit cast that is done implicitly due to the return value, use
a normal if statement instead of trying to squeeze everything together.

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