xfs: extract xfs_da_buf_copy() helper function

This patch does not modify logic.

xfs_da_buf_copy() will copy one block from src xfs_buf to
dst xfs_buf, and update the block metadata in dst directly.

Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>