xfs: decrease indenting problems in xfs_dabuf_map

Refactor the code that complains when a dir/attr mapping doesn't exist
but the caller requires a mapping.  This small restructuring helps us to
reduce the indenting level.

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