xfs: only flush the unshared range in xfs_reflink_unshare

There's no reason to flush an entire file when we're unsharing part of
a file.  Therefore, only initiate writeback on the selected range.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChandan Babu R <chandanrlinux@gmail.com>

fs/xfs/xfs_reflink.c

@@ -1502,7 +1502,8 @@ xfs_reflink_unshare(
 			&xfs_buffered_write_iomap_ops);
 	if (error)
 		goto out;
-	error = filemap_write_and_wait(inode->i_mapping);
+
+	error = filemap_write_and_wait_range(inode->i_mapping, offset, len);
 	if (error)
 		goto out;