xfs: split xfs_imap_to_bp

Split looking up the dinode from xfs_imap_to_bp, which can be
significantly simplified as a result.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>