libxfs: fix data corruption bug in libxfs_file_write

libxfs: fix data corruption bug in libxfs_file_write

libxfs_file_write tries to initialize the entire file block buffer,
which includes zeroing the head portion if @pos is not aligned to the
filesystem block size.  However, @buf is the file data to copy in at
position @pos, not the position of the file block.  Therefore, block_off
should be added to b_addr, not buf.

Cc: <linux-xfs@vger.kernel.org> # v6.13.0
Fixes: 73fb78e5ee8940 ("mkfs: support copying in large or sparse files")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>