xfs: fix xfs_reflink_unshare usage of filemap_write_and_wait_range

The final parameter of filemap_write_and_wait_range is the end of the
range to flush, not the length of the range to flush.

Fixes: 46afb062 ("xfs: only flush the unshared range in xfs_reflink_unshare")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>