xfs: clean up xfs_buf_item_get_format return value

The only thing that can cause a nonzero return from
xfs_buf_item_get_format is if the kmem_alloc fails, which it can't.
Get rid of all the unnecessary error handling.

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