xfs: simplify mappedbno handling in xfs_da_{get,read}_buf

Shortcut the creation of xfs_bmbt_irec and xfs_buf_map for the case
where the callers passed an already mapped xfs_daddr_t.  This is in
preparation for splitting these cases out entirely later.  Also reject
the mappedbno case for xfs_da_reada_buf as no callers currently uses
it and it will be removed soon.

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>